rtu: implement modbus sensor read

Read the modbus device config from the Entity Manager configuration and
create the sensor interfaces for related sensor register config.

Tested:
Added new Unit test named test_sensors -
```
> meson test -t 10 -C builddir/ --print-errorlogs --wrapper="valgrind --error-exitcode=1" test_sensors
ninja: Entering directory `/host/repos/Modbus/phosphor-modbus/builddir'
[2/2] Linking target tests/test_sensors
1/1 test_sensors        OK              13.98s

Ok:                1
Fail:              0
```

Tested on Qemu using Mock Modbus Device -
```
root@ventura:~# busctl tree xyz.openbmc_project.ModbusRTU
└─ /xyz
  └─ /xyz/openbmc_project
    ├─ /xyz/openbmc_project/inventory_source
    │ ├─ /xyz/openbmc_project/inventory_source/Heat_Exchanger_12_DevTTYUSB0
    │ ├─ /xyz/openbmc_project/inventory_source/Heat_Exchanger_12_DevTTYUSB1
    │ ├─ /xyz/openbmc_project/inventory_source/Reservoir_Pumping_Unit_12_DevTTYUSB0
    │ └─ /xyz/openbmc_project/inventory_source/Reservoir_Pumping_Unit_12_DevTTYUSB1
    └─ /xyz/openbmc_project/sensors
      └─ /xyz/openbmc_project/sensors/temperature
        ├─ /xyz/openbmc_project/sensors/temperature/Reservoir_Pumping_Unit_12_DevTTYUSB0_RPU_Coolant_Inlet_Temp_C
        ├─ /xyz/openbmc_project/sensors/temperature/Reservoir_Pumping_Unit_12_DevTTYUSB0_RPU_Coolant_Outlet_Temp_C
        ├─ /xyz/openbmc_project/sensors/temperature/Reservoir_Pumping_Unit_12_DevTTYUSB1_RPU_Coolant_Inlet_Temp_C
        └─ /xyz/openbmc_project/sensors/temperature/Reservoir_Pumping_Unit_12_DevTTYUSB1_RPU_Coolant_Outlet_Temp_C

busctl introspect xyz.openbmc_project.ModbusRTU /xyz/openbmc_project/sensors/temperature/Reservoir_Pumping_Unit_12_DevTTYUSB1_RPU_Coolant_Outlet_Temp_C
NAME                                TYPE      SIGNATURE RESULT/VALUE                             FLAGS
org.freedesktop.DBus.Introspectable interface -         -                                        -
.Introspect                         method    -         s                                        -
org.freedesktop.DBus.Peer           interface -         -                                        -
.GetMachineId                       method    -         s                                        -
.Ping                               method    -         -                                        -
org.freedesktop.DBus.Properties     interface -         -                                        -
.Get                                method    ss        v                                        -
.GetAll                             method    s         a{sv}                                    -
.Set                                method    ssv       -                                        -
.PropertiesChanged                  signal    sa{sv}as  -                                        -
xyz.openbmc_project.Sensor.Value    interface -         -                                        -
.MaxValue                           property  d         nan                                      emits-change writable
.MinValue                           property  d         nan                                      emits-change writable
.Unit                               property  s         "xyz.openbmc_project.Sensor.Value.Unit.… emits-change writable
.Value                              property  d         1670.6                                   emits-change writable
```

Change-Id: I1368e8df5999b5cee9ac19d185ee110a9ecc3021
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
diff --git a/tests/modbus_server_tester.hpp b/tests/modbus_server_tester.hpp
index c9b4416..ccfa204 100644
--- a/tests/modbus_server_tester.hpp
+++ b/tests/modbus_server_tester.hpp
@@ -19,18 +19,43 @@
 constexpr uint16_t testSuccessReadHoldingRegisterOffset = 0x0102;
 constexpr uint16_t testSuccessReadHoldingRegisterCount = 0x2;
 constexpr uint16_t testSuccessReadHoldingRegisterSegmentedOffset = 0x0103;
-constexpr std::array<uint16_t, testSuccessReadHoldingRegisterCount>
-    testSuccessReadHoldingRegisterResponse = {0x1234, 0x5678};
+const std::vector<uint16_t> testSuccessReadHoldingRegisterResponse = {
+    0x1234, 0x5678};
 constexpr uint16_t testFailureReadHoldingRegister = 0x0105;
 
 // Device Inventory Testing Constants
 constexpr uint16_t testReadHoldingRegisterModelOffset = 0x0112;
 constexpr uint16_t testReadHoldingRegisterModelCount = 0x8;
-constexpr std::array<uint16_t, testReadHoldingRegisterModelCount>
-    testReadHoldingRegisterModel = {0x5244, 0x4630, 0x3430, 0x4453,
-                                    0x5335, 0x3139, 0x0000, 0x3000};
+const std::vector<uint16_t> testReadHoldingRegisterModel = {
+    0x5244, 0x4630, 0x3430, 0x4453, 0x5335, 0x3139, 0x0000, 0x3000};
 constexpr std::string testReadHoldingRegisterModelStr = "RDF040DSS519";
 
+// Device Sensors Testing Constants
+constexpr uint16_t testReadHoldingRegisterTempCount = 0x1;
+constexpr uint16_t testReadHoldingRegisterTempUnsignedOffset = 0x0113;
+const std::vector<uint16_t> testReadHoldingRegisterTempUnsigned = {
+    0x0050}; // 80.0
+constexpr uint16_t testReadHoldingRegisterTempSignedOffset = 0x0114;
+const std::vector<uint16_t> testReadHoldingRegisterTempSigned = {
+    0xFFB0}; // -80.0
+
+static const std::map<uint16_t, std::tuple<uint16_t, std::vector<uint16_t>>>
+    testReadHoldingRegisterMap = {
+        {testSuccessReadHoldingRegisterOffset,
+         {testSuccessReadHoldingRegisterCount,
+          testSuccessReadHoldingRegisterResponse}},
+        {testSuccessReadHoldingRegisterSegmentedOffset,
+         {testSuccessReadHoldingRegisterCount,
+          testSuccessReadHoldingRegisterResponse}},
+        {testReadHoldingRegisterModelOffset,
+         {testReadHoldingRegisterModelCount, testReadHoldingRegisterModel}},
+        {testReadHoldingRegisterTempUnsignedOffset,
+         {testReadHoldingRegisterTempCount,
+          testReadHoldingRegisterTempUnsigned}},
+        {testReadHoldingRegisterTempSignedOffset,
+         {testReadHoldingRegisterTempCount, testReadHoldingRegisterTempSigned}},
+};
+
 class ServerTester
 {
   public: