Add per-connection client Initial DCID override

[mihed]

s2n-quic currently generates the client Initial Destination Connection ID internally in create_client_connection, and Connect has no API to override it. This blocks deployments that need client-chosen Initial DCID bits for QUIC SO_REUSEPORT steering before the server has connection state.

Request:
Add a public API like:

Connect::with_initial_destination_connection_id(InitialId)

Requirements:

• Validate QUIC Initial DCID length: 8..=20 bytes.
• Use the supplied ID for Initial packet keys, original_destination_connection_id, and initial peer connection ID.
• Keep default behavior unchanged when unset.
• Document that callers are responsible for RFC9000 unpredictability.

Use case:
With, for example, a custom 1byte prefix, you can steer to specific CPUs in the target via eBPF/BPF. This enables topology aware proxying.

How to apply after it lands:

let connect = Connect::new(runtime_addr)
    .with_server_name(runtime_name)
    .with_initial_destination_connection_id(dcid);

let connection = client.connect(connect).await?;

[maddeleine]

This is correct, s2n-quic does not let you configure the initial DCID on the client. It uses a random 8 byte ID for the client's initial DCID. s2n-quic does let you configure the source CID though, through the connection ID provider. Couldn't you steer using that value? I'm curious what type of steering you're trying to do here. If you want to send specific client's packets to specific CPUs on the server, I feel like that should already be possible.