WIP: Add KubernetesNamespaces as part of the payload

What does this MR do?

Add KubernetesNamespaces as part of the payload for agent_info private API endpoint. this change is backward compatible, only available for EE and is behind a feature flag.

This change makes kubernetes namespaces (only those related to the environments of a specific project) available within agentk.

Related issue: #277156 (closed)

This is the MR related changes related to agentk: gitlab-org/cluster-integration/gitlab-agent!252 (merged)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team