Implement gRPC for Request and Anchor events

gRPC on L1 is currently WiP and will be deployed at some point: https://github.com/iotaledger/iota/pull/7746/files

We therefore need to switch our current Websocket implementation with gRPC.

For that we need to take the protobuf files, generate Go types and use them inside a new gRPC client connector. Nodeconn should remain mostly untouched where possible.