Add constraint on services table

What does this MR do?

It adds a constraint to the services table to allow a service to be either a template or a project service

Service templates live in the same table as project services. When a new project is created, a copy of the template will be made and attached to the project. It should not be possible for a service to be a template and attached to a project at the same time.

We are adding a new column to the table soon (#204801 (closed)). When this gets added the constraint need to be changed to allow the service to be either template, instance or attached to a project.

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