Add namespace projects finder

Ref:

• #296665 (closed)
• #322917 (closed)

What does this MR do?

This MR introduces a new Namespaces::ProjectsFinder which will be consumed by our GraphQL API.

It includes the following items:

• Add new finder
• Add EE finder extension
• Move all logic from our current resolver to our new finder

This MR follows up the work introduced in !55383 (merged) and !55182 (merged).

Why are we doing this?

During the review of the previous MR mentioned above, it was brought up that a dedicated finder would be better than keep adding more logic to our existing graphql resolver.

By introducing this new finder, we are able to clean up some ~"technical debt" and allow a better readability and maintainer for future feature development.

Database review

Please note that we did not change our existing query introduced in !55383 (merged) & !55182 (merged).

We just moved our business logic to our dedicated finder.

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