Update client library for gRPC

Switch from protobuf-over-REST to gRPC wire format. Requests use
gRPC framing (5-byte length-prefix) with application/grpc+proto
content-type, routed to /api.LightningNode/{method}. HMAC auth
simplified to timestamp-only signing to match the server change.

Error handling uses Trailers-Only mode: when the server returns a
gRPC error without a body, grpc-status appears as a regular HTTP/2
header that reqwest can read.

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

Author: benthecarman