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.proto

@@ -540,6 +540,23 @@ service Lightning {
     */
     rpc CheckMacaroonPermissions (CheckMacPermRequest)
         returns (CheckMacPermResponse);
+
+    /*
+    RegisterRPCMiddleware adds a new gRPC middleware to the interceptor chain. A
+    gRPC middleware is software component external to lnd that aims to add
+    additional business logic to lnd by observing/intercepting/validating
+    incoming gRPC client requests and (if needed) replacing/overwriting outgoing
+    messages before they're sent to the client. When registering the middleware
+    must identify itself and indicate what custom macaroon caveats it wants to
+    be responsible for. Only requests that contain a macaroon with that specific
+    custom caveat are then sent to the middleware for inspection. The other
+    option is to register for the read-only mode in which all requests/responses
+    are forwarded for interception to the middleware but the middleware is not
+    allowed to modify any responses. As a security measure, _no_ middleware can
+    modify responses for requests made with _unencumbered_ macaroons!
+    */
+    rpc RegisterRPCMiddleware (stream RPCMiddlewareResponse)
+        returns (stream RPCMiddlewareRequest);
 }
 
 message Utxo {