Supports connection pool reuse in http2 proxy (nginx/envoy)

[godhth]

Is your feature request related to a problem? Please describe.
当使用http2（nginx/envoy）代理时，由于网关使用了连接池管理复用连接，而nacos仅支持单个client 使用单个连接，导致重复的连接无法建立双向流。

When using the http2 (nginx/envoy) proxy, because the gateway uses a connection pool to manage multiplexed connections, nacos only supports a single client to use a single connection, resulting in repeated connections that cannot establish a bidirectional stream.

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[KomachiSion]

Nacos的连接是有意义的， 被代理复用后可能会导致数据串掉， 这个是怎么解决的？

[KomachiSion]

这个改动很大，需要完整的设计一下。

[godhth]

数据是通过不同stream来区分的，复用一个连接，但是streamId不同，每个streamObserver不同，数据不会串。
现在的问题是复用的连接，nacos认为是重复的，没有保存到ConnectionManager，因为当前的connectionId只能代表实际的物理连接，复用这种情况其实是逻辑连接。
所以我一开始想的是给connectionId 加上双向流的streamId的属性 来达到逻辑连接的效果，但是单向流获取不到双向流的streamId，单向流里也需要使用connectionId ，所有放弃了这个想法，进而使用给客户端增加一个标识来达到类似的目的，

实现了逻辑连接的效果，那么复用的连接也会被正确register 到ConnectionManager中，每个逻辑连接注册的streamObserver是不同的，所以数据不会串。

[godhth]

[KomachiSion]

@shiyiyue1102 help to review this design.

[KomachiSion]

This feature is a big changes for nacos connection between server and client.

It should be an experimental feature, If want to do this, Please make sure following:

1. do code abstract, make your add feature code is independent and make sure remove them will not cause current connection way problem.
2. add switch for this feature and default false.