Add batched delivery wire formats to streaming destinations

Huzaifa Iftikharrequested to merge
nats-05-batched-destinations into master

What does this MR do and why?

Add a batch delivery path alongside the existing per-event stream:

  • BaseStreamDestination.for_batch(destination) builds a batch-mode streamer, and #stream_batch(event_bodies) is the batch entry point.
  • HTTP: POST a JSON array of event bodies; drop the per-request X-Gitlab-Audit-Event-Type header (each element self-describes type).
  • GCP: map the batch to a multi-entry Cloud Logging write.
  • S3: write one object per batch, keyed YYYY/MM/DD/<time_in_ms>_<first_id>.
  • request_body now uses the upstream stream_id (falling back to a UUID) so stream-only events keep a stable id across destinations and transports.

HTTP and S3 batched formats are breaking and gated per-group by the audit_event_streaming_via_nats flag; the per-event path is unchanged.

Closes - #605042 (closed) Part of epic &17582

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