Kubernetes Agent extensibility

Make it easy to extend GitLab Kubernetes Agent (agentk) and/or GitLab Kubernetes Agent Server (gitlab-kas) with new features. This should help other teams to contribute new features and improvements.

This work involves multiple things:

• Design the new structure for the agent codebase to make it easier to extend. https://gitlab.com/gitlab-org/cluster-integration/gitlab-agent/-/blob/master/doc/modules.md
• Refactor the agent codebase to follow the above design. gitlab-org/cluster-integration/gitlab-agent#70 (closed)
• Design the overall extensibility approach. !48320 (closed)
• Implement the consolidated endpoint in the rails backend.
• Document how to extend the agent.
  • How to add new configuration for module
  • Document security considerations when extending the agent, link to #287710