Add support for post code handling/eventing

Add support to consume a JSON configuration which
will let a platform configuration to dictate if
we need to do special handling or logging based
on specific post codes received.

Tested:

1. Created the following config and after restarting
`xyz.openbmc_project.State.Boot.PostCode@0.service`:
```
[
    {
        "primary": [1,2,5],
        "event": {
            "name": "xyz.openbmc_project.State.SMC.SMCFailed",
            "arguments": {
                "IDENTIFIER": "test",
                "FAILURE_TYPE": "world"
            }
        }
    }
]
```

Created the primary post code:
```
root@bmc:~# busctl set-property xyz.openbmc_project.State.Boot.Raw /xyz/openbmc_project/state/boot/raw0 xyz.openbmc_project.State.Boot.Raw Value '(ayay)' 3 0x1 0x2 0x5 0x0
```
Check that the log is created:
```
root@bmc:~# busctl introspect -l xyz.openbmc_project.Logging /xyz/openbmc_project/logging/entry/2 xyz.openbmc_project.Logging.Entry | grep "AdditionalData\|Message"
.AdditionalData property  a{ss}     6 "FAILURE_TYPE" "world" "IDENTIFIER" "test" <snip>
.Message property  s "xyz.openbmc_project.State.SMC.SMCFailed"
```
2. Replace the config with:
```
[
    {
        "primary": [1,2,6],
        "targets": ["testservice.service"]
    }
]
```
Create a test unit file:
testservice.service:
```
[Unit]
Description=A simple oneshot service

[Service]
Type=oneshot
ExecStart=/bin/bash -c "echo Hello world > /tmp/test.txt"
```
Issue a new post code with the new config.
```
systemctl daemon-reload
systemctl restart xyz.openbmc_project.State.Boot.PostCode@0.service
root@bmc:/usr/lib/systemd/system# busctl set-property xyz.openbmc_project.State.Boot.Raw /xyz/openbmc_project/state/boot/raw0 xyz.openbmc_project.State.Boot.Raw Value '(ayay)' 3 0x1 0x2 0x5 0x0
root@bmc:/usr/lib/systemd/system# cat /tmp/test.txt
Hello world
```

Change-Id: Ibaa09fb88190a344fd7c4fadd24dda4aa4ae0633
Signed-off-by: Amithash Prasad <amithash@meta.com>
diff --git a/meson.build b/meson.build
index c7e9e16..dbb94d6 100644
--- a/meson.build
+++ b/meson.build
@@ -7,6 +7,8 @@
     version: '1.0',
 )
 
+# Disable JSON implicit conversions.
+add_project_arguments('-DJSON_USE_IMPLICIT_CONVERSIONS=0', language: 'cpp')
 
 conf_data = configuration_data()
 conf_data.set_quoted(
@@ -33,6 +35,7 @@
 sdbusplus = dependency('sdbusplus')
 phosphor_logging = dependency('phosphor-logging')
 phosphor_dbus_interfaces = dependency('phosphor-dbus-interfaces')
+json = dependency('nlohmann_json', include_type: 'system')
 
 cxx = meson.get_compiler('cpp')
 cereal_dep = dependency('cereal', required: false)
@@ -66,6 +69,14 @@
     strip_directory: true,
 )
 
+packagedir = join_paths(
+    get_option('prefix'),
+    get_option('datadir'),
+    'phosphor-post-code-manager',
+)
+
+install_data(sources: 'post-code-handlers.json', install_dir: packagedir)
+
 executable(
     'post-code-manager',
     'src/main.cpp',
@@ -76,6 +87,7 @@
         phosphor_dbus_interfaces,
         phosphor_logging,
         cereal_dep,
+        json,
     ],
     include_directories: 'inc',
 )