Service validation for project and type uniqueness

What does this MR do?

This adds a validation on Service for uniqueness of type and project_id.

As discussed in #207385 (closed) we can only validate this on :create because it is possible that the database holds invalid data. For now, the validation only prevents new invalid services to be added.

How to test on Staging

It is not possible to violate the validation on the UI. On Staging we can try if creating and updating a project service still works.

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

Closes #207385 (closed)