lnrpc/walletrpc: implement the WalletKitServer gRPC service

In this commit, we implement the newly defiend WalletKitServer gRPC
service. We use the same template w.r.t build tags as the existing
signrpc service.

lnrpc/walletrpc/config_default.go

@@ -0,0 +1,8 @@
+// +build !walletrpc
+
+package walletrpc
+
+// Config is the primary configuration struct for the WalletKit RPC server.
+// When the server isn't active (via the build flag), callers outside this
+// package will see this shell of a config file.
+type Config struct{}