[OpenOLT Agent] Measure and read Rx optical power for an ONU

The rx optical power info residing on an OLT PON transceiver holds the last measured value for a specific ONU behind that PON port. In order to read it, we must first trigger RSSI measurement through PON MAC subsystem. After getting the corresponding indication, we read the value directly from the transceiver.

In this task, we will open an API on openolt.proto and it will synchronously return dBm value to Voltha (it blocks until the measurement is completed, reads the eeprom value, converts the value to dBm and returns the converted value).