remote signer grpc server

[agouin]

Add additional service to accept privval connections over grpc rather than connect out.
Existing functionality remains the same, this is not a breaking change.

GRPC service is enabled by setting a non-empty grpcAddr string in the config.yaml. This will be used as the listen address. Example: :5555, 0.0.0.0:5555

Service API is minimal and excludes any comet types

service RemoteSigner {
	rpc PubKey (PubKeyRequest) returns (PubKeyResponse) {}
	rpc Sign(strangelove.horcrux.SignBlockRequest) returns (strangelove.horcrux.SignBlockResponse) {}
}

Other:

• Bump go to 1.21
• Switch to buf for proto-gen