65113 add CI_PROJECT_NAMESPACE_SLUG variable and a test for the build_spec

What does this MR do?

This MR add CI_PROJECT_NAMESPACE_SLUG into project model. Can be used for example Kubernetes Namespaces als Group releation. Risk should be very minimal.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Code review guidelines
• Merge request performance guidelines
• Style guides

Performance and testing

No risks for performance.

Security

Should be no Security issues.

Closes #65113 (moved)