Fixes#5307.
Corrects some typos in the REST annotations for the routerrpc subserver.
Unfortunately these mistakes aren't caught by the current version of the
grpc-gateway library. But we intend to update to v2 soon which will
output alerts as mentioned in #5307.
The grpc-gateway library that is used to transform REST calls into gRPC
uses a different method for reading a request body stream depending on
whether the RPC is a request-streaming one or not. We can't really find
out what kind of RPC the user is calling at runtime, so we add a new
parameter to the proxy that lists all request-streaming RPC calls.
In any case the client _has_ to send one request message initially to
kick off the request processing. Normally this can just be an empty
message. This can lead to problems if that empty message is not
expected by the gRPC server. But for the currently existing two
client-streaming RPCs this will only trigger a warning
(HTLC interceptor) or be ignored (channel acceptor).
As a convenience method for users to look up what RPC method URIs exist
and what permissions they require, we add a new ListPermissions call
that simply returns all registered URIs (including internal and external
subservers) and their required permissions.
In this commit we add the ability to intercept forwarded htlc packets
straight from the RPC layer. The RPC layer handles a bidrectional stream
that comminucates to the client the intercepted packets and handles its
response by coordinating with the interceptable switch.
