Allow specifying Kubernetes namespace via CI

What does this MR do?

Add Kubernetes option to environment CI config

Introduces a new CI configuration option that allows specifying the Kubernetes namespace that an environment will be deployed to. This option does not currently have any effect on deployments, this will be added in a separate merge request

Example usage:

    environment:
      name: production
      url: https://production.example.com
      kubernetes:
        namespace: production

Screenshots

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

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

#27630 (closed)