Add gRPC service definition to api.proto

Define the LightningNode service with all 38 unary RPCs and a
server-streaming SubscribeEvents RPC. Add SubscribeEventsRequest
message and events.proto import. Update response comments from
REST/HTTP style to gRPC style.

prost-build ignores service blocks, so this is a documentation-
only change that prepares for the gRPC migration without altering
any generated code or runtime behavior.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: benthecarman