[Feature]Support advanced http router and features

[lizzzcai]

1. use mux as the default handler in ff-go which is more production-ready and we can bring in new features - Create Functions Framework Spec functions-framework#21 (comment), Serve multipe routes on the same function functions-framework#17
2. support defining variables in the path like /functions/<funcname>, and use it in the function by ofnctx.Vars()
3. support defining method matcher like withFunctionMethods("GET", "POST")