lnrpc: split REST annotations into service files

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.

lnrpc/watchtowerrpc/watchtower.yaml

@@ -0,0 +1,7 @@
+type: google.api.Service
+config_version: 3
+
+http:
+  rules:
+    - selector: watchtowerrpc.Watchtower.GetInfo
+      get: "/v2/watchtower/server"