Add the new RPC method that looks up the base scid for a given alias.
Given the previous stepping stones this commit is fairly simple, we just
call into the alias manager and return the lookup result.
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.