pcie_bifurcation: Fetch bifurcation at a PCIe slot with hardcoded values

This will read a json config and return the bifurcation based on that.
It will read the configure file in a persistent file to see if it is
valid machine config and return the bifurcation information
accordingly.

For example,
If it is valid config, it will return 8x8 for PE1,3,4,6.

Tested:
Unit tests passed.

Physical Tests.

PE0 -> no bifurcation
PE1 -> x8x8

```
$ ipmitool raw 0x2e 0x32 0x79 0x2b 0x00 0x0f 0
 79 2b 00 0b 00

$ ipmitool raw 0x2e 0x32 0x79 0x2b 0x00 0x0f 1
 79 2b 00 0b 02 08 08
```

Change-Id: I44aefbfb26372f8bc0069343c8a6d07d3cf6f42d
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/handler.hpp b/handler.hpp
index 8c36c52..d919f05 100644
--- a/handler.hpp
+++ b/handler.hpp
@@ -187,6 +187,14 @@
      */
     virtual void accelOobWrite(std::string_view name, uint64_t address,
                                uint8_t num_bytes, uint64_t data) const = 0;
+
+    /**
+     * Prase the I2C tree to get the highest level of bifurcation in target bus.
+     *
+     * @param[in] index    - PCIe Slot Index
+     * @return list of lanes taken by each device.
+     */
+    virtual std::vector<uint8_t> pcieBifurcation(uint8_t index) = 0;
 };
 
 } // namespace ipmi