The standard of Http 2 Dubbo transcoding

- [x] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [x] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2 and 3
* Operating System version: xxx
* Java version: xxx

### Steps to reproduce this issue

Recently, I work on the http2dubbo transcoding, i find there is no unified specification that describe how to make a http request, how to pass the service and method to transcoder, ... can we make the http 2 dubbo invocation base on the unified standard ? may be we can have a discussion about this, and this work may help gateway developer to implement their own transcoder, and users may use http2dubbo transcoder more conveniently.

Pls. provide [GitHub address] to reproduce this issue.

### Expected Result
a unifed standard for transforming http request to dubbo request(and triple), also response !

### Actual Result