lnrpc/signrpc: add lnrpc.SubServerDriver for signrpc

In this commit, we create a lnrpc.SubServerDriver for signrpc. Note that
this file will only have its init() method executed if the proper build
flag is on. As a result, only if the build flag is set, will the RPC
server be registered, and visible at the packge lnrpc level for the root
server to manipulate.

lnrpc/signrpc/config_default.go

@@ -1,10 +1,6 @@
-// +build !signerrpc
+// +build !signrpc
 
 package signrpc
 
-// Config is the primary configuration struct for the signer RPC server. It
-// contains all the items required for the signer rpc server to carry out its
-// duties. The fields with struct tags are meant to be parsed as normal
-// configuration options, while if able to be populated, the latter fields MUST
-// also be specified.
+// Config is empty for non-signrpc builds.
 type Config struct{}