System-level Validation for CI/CD templates [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR addresses Validate CI/CD Templates at system-level for aligning the behaviors, specifically:

• For template authors: Allow CI/CD templates to declare its metadata.
• For template consumers: When a template is included at pipeline creation, the system performs run-time validation to verify the template is safe to be used.

This feature is behind ci_template_metadata feature flag.

Related #326298

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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