Monitor the optical links RX power

Summary

Both during commissioning and operations, monitoring the RX power from each of the optical links is a precious debugging ally. Currently, the only way to read this value is through some Python scripts in some CTP7.

This issue consists in two parts:

• Write a RPC method implementing such a feature
• Write the readout and display code in gemmonitor

What is the expected correct behavior?

One should be able to monitor the optical links RX power.