Support gNMI Set RPC.

[robshakir]

Allow the device configuration to be updated during the lifecycle of the device with the Set RPC.

[robshakir]

This should be implemented outside of the gRIBIgo fake as part of wider fake operations.

[robshakir]

Partial support is needed here in order to allow non-default network instance tests to be run.

[robshakir]

#115 adds specific methods to do this rather than use gNMI set. Closing this issue.