Dubbo GSoC 2023 - Refactor the http layer

## Background
Dubbo currently supports the rest protocol based on http1, and the triple protocol based on http2, but currently the two protocols based on the http protocol are implemented independently, and at the same time, they cannot replace the underlying implementation, and their respective implementation costs are relatively high.

## Target
In order to reduce maintenance costs, we hope to be able to abstract http. The underlying implementation of the target implementation of http has nothing to do with the protocol, and we hope that different protocols can reuse related implementations.

https://issues.apache.org/jira/browse/DUBBO-112

Start by reading the independent REST-over-HTTP/1 and triple-over-HTTP/2 implementations and compare the underlying HTTP responsibilities. Define an abstraction that is independent of the protocol and allows the underlying implementation to be replaced; done means both protocols can reuse it while preserving their respective behavior.

Written by the indexing model from the issue text.