Remove gRPC as a hard dependency

[fusion44]

Currently gRPC is always installed, even if the user only runs Core Lightning.

The dependencies for the lightning backend must be installed by pip manually upon install:

• LND: pip install grpcio grpcio-tools googleapis-common-protos protobuf
• CLN: pip install pyln-client

[rootzoll]

Maybe gRPC can now be of bigger use with new Core Lightning supports this interface?

[fusion44]

Yes, but the end goal is to extract the abstraction layer to its own library. I want to make it usable for LNbits medium term too. LNbits supports also other funding sources that don't have a gRPC requirement.