Support SOCKS5 proxy for tri protocol in NettyConnectionClient

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### Apache Dubbo Component

Java SDK (apache/dubbo)

### Descriptions

**Body:**
Hello maintainers,

We are currently using Dubbo **3.3.6** with the **tri (Triple) protocol**. While reviewing the source code, I noticed that `org.apache.dubbo.remoting.transport.netty4.NettyConnectionClient` still contains the following TODO:

```java
// set null but do not close this client, it will be reconnecting in the future
ch.closeFuture().addListener(channelFuture -> clearNettyChannel());
// TODO support Socks5
```

From our current testing, it seems that **tri protocol does not yet support SOCKS5 proxying** on the client side. JVM-level proxy settings such as:

```bash
-DsocksProxyHost=...
-DsocksProxyPort=...
```

do not appear to work for tri connections, which is understandable since the client is based on **Netty** rather than plain JDK sockets.

We have a real use case for this feature, such as:
- local development environments accessing remote Dubbo services through a SOCKS5 proxy
- cross-network or restricted-network scenarios where direct connectivity is not available

Could you please clarify:

1. Is **SOCKS5 support for tri protocol** already on the roadmap?
2. If yes, is there any **planned version or milestone** for it?
3. If not, what would be the **recommended extension point or workaround** for now?
For example, is it expected to support something like Netty's `Socks5ProxyHandler` in the client pipeline in the future?

Environment:
- Dubbo version: **3.3.6**
- Protocol: **tri / Triple**
- Transport: **Netty**

Thanks in advance.

### Related issues

_No response_

### Are you willing to submit a pull request to fix on your own?

- [x] Yes I am willing to submit a pull request on my own!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Start with org.apache.dubbo.remoting.transport.netty4.NettyConnectionClient and its TODO for SOCKS5 support, then review how tri protocol client connections build the Netty pipeline. Done means determining and implementing client-side SOCKS5 proxy support for tri connections, with verification that connections work through a SOCKS5 proxy.

Written by the indexing model from the issue text.