feat: Batched write API for latency optimization

[aparatext]

When forwarding multiple packets immediately (e.g., proxy use case), there's no way to batch writes before TCP flush. With TCP_NODELAY enabled (as in examples), each Connection::write() call sends a separate TCP segment.

Proposed API:

conn.write_batch(&[packet1, packet2]).await?;`

or

conn.begin_batch();
conn.write(packet1).await?;
conn.write(packet2).await?;
conn.flush_batch().await?;

This would help proxies/servers optimize connection handshakes and burst scenarios.

[mat-1]

Sure, this seems reasonable. I think your first proposed approach would be the simplest way.

[suprohub]

Maybe add and first and second api together? sometimes better 1 sometimes better 2