Add grpc interceptors that inject an LNC session's ID into the context
as gRPC metadata. By injecting it as such, it will be transported over
the wire in any outgoing gRPC calls.
This lets us be sure that any session call sent to the RPCMiddleware
interceptor in LND will continue to be grouped along with the
appropriate session ID. This gives LND a way to send the metadata we
include back to LiT meaning that we will later on be able to extract the
session ID again.
