Adding two tests for 400ZR chromatic dispersion and eSNR (openconfig#…

…2236)

* Add tests for 400ZR chromatic dispersion and eSNR.

feature/platform/transceiver/zr_esnr_and_cd_test/README.MD

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+# gNMI-1.20: Telemetry: 400ZR Electrical Signal to Noise Ratio(eSNR) and Chromatic Dispersion(CD) telemetry values streaming
+
+## Summary
+
+Validate 400ZR optics module reports accurate eSNR and CD telemetry values.
+
+eSNR is defined as the electrical Signal to Noise ratio at the decision
+sampling point in dB
+
+Chromatic Dispersion is frequency dependent change in signal phase velocity due
+to fiber measured in ps/nm 
+
+## 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. Connection
+    between the modules should pass through an optical switch that can be
+    controlled through automation to simulate a fiber cut.  
+*   To establish a point to point ZR link ensure the following:
+      * Both transceivers states are 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 ZR link is 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/esnr/instant
+    *   /terminal-device/logical-channels/channel/otn/state/esnr/avg
+    *   /terminal-device/logical-channels/channel/otn/state/esnr/min
+    *   /terminal-device/logical-channels/channel/otn/state/esnr/max
+    *   /platform/components/component/optical-channel/state/chromatic-dispersion/instant
+    *   /platform/components/component/optical-channel/state/chromatic-dispersion/avg
+    *   /platform/components/component/optical-channel/state/chromatic-dispersion/min
+    *   /platform/components/component/optical-channel/state/chromatic-dispersion/max
+
+*   When the modules or the devices are still in a boot stage, they must not
+    stream any invalid string values like "nil" or "-inf" until valid values
+    are available for streaming.
+
+*   eSNR and CD streamed values must always be of type Decimal64.
+    When link interfaces are in down state 0 must be reported as a valid
+    value.
+
+**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 eSNR and CD is updated after the interface flaps.
+
+    *   Enable a pair of ZR interfaces on the DUT as explained above.
+    *   Verify the ZR optics eSNR and CD telemetry values are in the normal range.
+    *   Disable or shut down the interface on the DUT.
+    *   Verify with interfaces in down state both optics are streaming Decimal64 0
+        value for both eSNR and CD.
+    *   Re-enable the interfaces on the DUT.
+    *   Verify the ZR optics eSNR and CD telemetry values are updated to the
+        value in the normal range again.
+        * Typical expected value range for eSNR is 13.5 to
+          18 dB +/-0.1 dB.
+        * Typical CD expected value range is 0 to 2400 ps/nm.
+
+*   Verify that the optics eSNR and CD is updated after a fiber cut.
+
+    *   Enable a pair of ZR interfaces on the DUT as explained above.
+    *   Verify the ZR optics eSNR and CD telemetry values are in the normal
+        range.
+    *   Simulate a fiber cut using the optical switch that sits in-between the
+        DUT ports.
+    *   Verify with link in down state due to fiber cut both optics are streaming
+        Decimal64 0 value for both eSNR and CD.
+    *   Re-enable the optical switch connection to clear the fiber cut fault.
+    *   Verify the ZR optics eSNR and CD telemetry values are updated to the value in the normal
+        range again.
+        * Typical expected value range for eSNR is 13.5 to
+          18 dB +/-0.1 dB.
+        * Typical CD expected value range is 0 to 2400 ps/nm.
+
+## Config Parameter coverage
+
+*   /components/component/transceiver/config/enabled
+
+## Telemetry Parameter coverage
+
+*   /terminal-device/logical-channels/channel/otn/state/esnr/instant
+*   /terminal-device/logical-channels/channel/otn/state/esnr/avg
+*   /terminal-device/logical-channels/channel/otn/state/esnr/min
+*   /terminal-device/logical-channels/channel/otn/state/esnr/max
+*   /platform/components/component/optical-channel/state/chromatic-dispersion/instant
+*   /platform/components/component/optical-channel/state/chromatic-dispersion/avg
+*   /platform/components/component/optical-channel/state/chromatic-dispersion/min
+*   /platform/components/component/optical-channel/state/chromatic-dispersion/max