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.
As a preparation for the migration to the grpc-gateway/v2 library we
declare each service's REST annotations in its own file. This is
optional in the v1 library but mandatory in v2.
George Tsagkarelis