Dubbo GSoC 2023 - Refactor Connection

## Background
At present, the abstraction of connection by client in different protocols in Dubbo is not perfect. For example, there is a big discrepancy between the client abstraction of connection in dubbo and triple protocols. As a result, the enhancement of connection-related functions in the client is more complicated, and the implementation cannot be reused. At the same time, the client also needs to implement a lot of repetitive code when extending the protocol.

## Target
Reduce the complexity of the client part when extending the protocol, and increase the reuse of connection-related modules.

https://issues.apache.org/jira/browse/DUBBO-113

Start by reviewing the issue background and the linked DUBBO-113 ticket, then trace how Dubbo and triple clients currently abstract connections. Compare the protocol-specific implementations and identify the repeated connection-related code. Done means reducing client complexity for protocol extensions and enabling reuse of connection-related modules.

Written by the indexing model from the issue text.