multi: add RPC middleware interception

With the middleware handler in place, we now need to add a new gRPC
interceptor to the interceptor chain that will send messages to the
registered middlewares for each event that could be of interest to them.

lnrpc/lightning.yaml

@@ -153,4 +153,6 @@ http:
     - selector: lnrpc.Lightning.CheckMacaroonPermissions
       post: "/v1/macaroon/checkpermissions"
       body: "*"
-
+    - selector: lnrpc.Lightning.RegisterRPCMiddleware
+      post: "/v1/middleware"
+      body: "*"