Add support for HAProxy Protocol ("proxy protocol")

Is there a way to add a CustomHandler to ChannelPipeline using NettyServerBuilder ?

I initialize my GrpcNettyServer by below:

```
NettyServerBuilder serverBuilder =
NettyServerBuilder
.forPort(config.getPort())
.addService();
.intercept();
.executor();
.build();
```
All I want is to get the channel from the server and add a custom handler to the channel pipeline.
Does NettyServerBuilder supports that option ?

Start by reading NettyServerBuilder and the issue discussion to determine how GrpcNettyServer exposes its ChannelPipeline and whether CustomHandler integration is supported. Clarify the intended HAProxy Protocol behavior and identify the relevant tests; done means the requested proxy-protocol support or pipeline customization is covered and verified.

Escrito por el modelo de indexación a partir del texto del issue.