rpc, p2p/simulations: use github.com/gorilla/websocket (#20289)

* rpc: improve codec abstraction

rpc.ServerCodec is an opaque interface. There was only one way to get a
codec using existing APIs: rpc.NewJSONCodec. This change exports
newCodec (as NewFuncCodec) and NewJSONCodec (as NewCodec). It also makes
all codec methods non-public to avoid showing internals in godoc.

While here, remove codec options in tests because they are not
supported anymore.

* p2p/simulations: use github.com/gorilla/websocket

This package was the last remaining user of golang.org/x/net/websocket.
Migrating to the new library wasn't straightforward because it is no
longer possible to treat WebSocket connections as a net.Conn.

* vendor: delete golang.org/x/net/websocket

* rpc: fix godoc comments and run gofmt

rpc/inproc.go

@@ -26,8 +26,8 @@ func DialInProc(handler *Server) *Client {
 	initctx := context.Background()
 	c, _ := newClient(initctx, func(context.Context) (ServerCodec, error) {
 		p1, p2 := net.Pipe()
-		go handler.ServeCodec(NewJSONCodec(p1), OptionMethodInvocation|OptionSubscriptions)
-		return NewJSONCodec(p2), nil
+		go handler.ServeCodec(NewCodec(p1), 0)
+		return NewCodec(p2), nil
 	})
 	return c
 }