Add publish_events to Gitlab::Kas::Client for events platform

What does this MR do and why?

Adds Gitlab::Kas::Client#publish_events to support publishing CloudEvents to the events_platform service on GitLab Relay (KAS).

References

Related to #594102 (closed)

Roll out issue #601527

Screenshots or screen recordings

How to set up and validate locally

1. In your GDK directory run

   gdk config set gitlab_k8s_agent.enabled true
   gdk reconfigure

   If it's not yet enabled in gdk.yml
2. Edit gitlab-k8s-agent-config.yml file and add events_platform: {} at the end.
3. Run gdk restart
4. The simplest way to check whether events received is to use relay-playground. Follow "Quick start" steps to configure. Use 2nd terminal window for that.
5. Start Relay Playground TUI.
6. In the TUI, hit ctrl+p to open settings. Set the Subscribe/Watch topic to test.publish.
7. Make sure the Subscribe and Watch panes are visible (toggle via the commands menu if not).
8. Hit ctrl+l to start the subscribe stream.
9. In the 1st terminal window, from rails directory, run gdk rails c.
10. Enable feature flag Feature.enable(:publish_events_to_relay)
11. Create and publish an event

    event = Gitlab::Agent::Event::CloudEvent.new(
      id: SecureRandom.uuid,
      source: 'rails-test',
      spec_version: '1.0',
      type: 'com.example.test',
      text_data: { hello: 'world' }.to_json
    )
    
    Gitlab::Kas::Client.new.publish_events(topic: 'test.publish', events: event)

The command should return event ID => ["1747840000-0"] (actual ID will differ). Relay playground should display received events.

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