RATIS-2625. Streaming: support control plane in Data Stream

[amaliujia]

What changes were proposed in this pull request?

This PR is AI-assisted

Adds a first-class mid-stream command plane to Ratis data streams. Callers can send opaque command payloads during an active stream via DataStreamOutput.commandAsync(). The server delivers them to StateMachine.DataStream.onCommand() instead of writing bytes to the data channel. Commands are ordered with data writes, replicated to remote peers like STREAM_DATA, and do not advance the stream byte offset.

Example usage:

// Client
out.getHeaderFuture().join();
out.writeAsync(dataBuffer).join();
out.commandAsync(myCommandBytes).join();  // opaque payload
out.writeAsync(moreData, StandardWriteOption.CLOSE).join();

// State machine
@Override
public CompletableFuture<?> onCommand(ByteBuffer command, long streamOffset) {
  // parse command, e.g. force(), metadata update, etc.
  return CompletableFuture.completedFuture(null);
}

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/RATIS-2625

How was this patch tested?

Unit tests

[amaliujia]

@szetszwo this is the follow up for apache/ozone#10823 (comment)