Add nats-pure gem, Gitlab::Nats config, client, and app setting

Huzaifa Iftikharrequested to merge
nats-01-foundation into master

What does this MR do and why?

Add nats-pure gem, Gitlab::Nats config, client, and app setting

Introduces the foundation for NATS-based audit event streaming:

  • Add the nats-pure gem (JetStream client).
  • Add Settings.nats (servers, TLS, connect_timeout, stream_replicas) and the config/gitlab.yml.example block. Empty servers means NATS is not configured and callers fall back to their non-NATS paths.
  • Add Gitlab::Nats (configured?/enabled? gates) and Gitlab::Nats::Client, a thin nats-pure wrapper (publish, pull_subscribe, stream management).
  • Add the use_nats_for_audit_streaming instance application setting (the operator master switch) with its JSON schema, model accessor, and Application Settings REST API exposure.

Closes #604448 (closed)

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Before After

How to set up and validate locally

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