In this commit, we introduce a new sub-package within the greater RPC
package. This new sub-package will house a new set of sub-RPC servers
to expose experimental features behind build flags for upstream
consumers. In this commit, we add the first config for the service,
which will simply expose the lnwallet.Signer interface over RPC.
In the default file, we have what the config will be if the build tag
(signerrpc) is off. In this case, the config parser won't detect any
times, and if specified will error out. In the active file, we have the
true config that the server will use. With this new set up, we'll
exploit these build flags heavily in order to create a generalized
framework for adding additional sub RPC servers.
