builder for EmbeddedChannel

[Bue-von-hon]

If we provide a builder instead of a constructor, it's easier to implement feature need to add parameters.
If you are ok, I'll create a PR and send it to you. 😄

target file https://github.com/netty/netty/blob/4.1/transport/src/main/java/io/netty/channel/embedded/EmbeddedChannel.java

Expected behavior

var channel = EmbeddedChannel.Builder()
                                        .setChannelId(...)
                                        .setHasDisconnect(...)
                                        .build();