400ZR test for monitoring pre FEC BER. (openconfig#2078)

* 400ZR transceiver test for monitoring pre-FEC BER.

---------

Co-authored-by: Darren Loher <[email protected]>

feature/platform/transceiver/ZR_pre-fec_ber_test/README.md

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+# gNMI-1.20: Telemetry: 400ZR Optics Pre-FEC(Forward Error Correction) BER(Bit Error Rate)
+
+## Summary
+
+Validate 400ZR optics module reports pre-FEC bit error rate performance data.
+
+## Procedure
+
+*   Connect two ZR interfaces using a duplex LC fiber jumper such that TX
+    output power of one is the RX input power of the other module.
+*   To establish a point to point ZR link ensure the following:
+      * Both transceivers state is enabled
+      * Both transceivers are set to a valid target TX output power
+        example -10 dBm
+      * Both transceivers are tuned to a valid centre frequency
+        example 193.1 THz
+*   With the link ZR link established as explained above, verify that the
+    following ZR transceiver telemetry paths exist and are streamed for both
+    the ZR optics
+    *   /terminal-device/logical-channels/channel/otn/state/pre-fec-ber/instant
+    *   /terminal-device/logical-channels/channel/otn/state/pre-fec-ber/avg
+    *   /terminal-device/logical-channels/channel/otn/state/pre-fec-ber/min
+    *   /terminal-device/logical-channels/channel/otn/state/pre-fec-ber/max
+
+**Note:** For min, max, and avg values, 10 second sampling is preferred. If 
+          10 seconds is not supported, the sampling interval used must be
+          communicated.
+
+
+*   Verify that the optics pre-FEC BER is updated after the interface flaps.
+
+    *   Enable a pair of ZR interfaces on the DUT as explained above.
+    *   Verify the ZR optics pre FEC BER PMs are in the normal range.
+    *   Disable or shut down the interface on the DUT.
+    *   Re-enable the interfaces on the DUT.
+    *   Verify the ZR optics pre FEC PM is updated to the value in the normal
+        range again. Typical expected value should be less than 1.2E-2
+
+## Config Parameter coverage
+
+*   /components/component/oc-transceiver:transceiver/oc-transceiver/config/enabled
+
+## Telemetry Parameter coverage
+
+    *  /terminal-device/logical-channels/channel/otn/state/pre-fec-ber/instant
+    *  /terminal-device/logical-channels/channel/otn/state/pre-fec-ber/avg
+    *  /terminal-device/logical-channels/channel/otn/state/pre-fec-ber/min
+    *  /terminal-device/logical-channels/channel/otn/state/pre-fec-ber/max