Fix N+1 in /projects REST endpoint

What does this MR do?

In this MR we are fixing two N+1 in the /projects REST endpoint that are really coupled together.

The first one is with saml_providers. This is the look of the query regarding that field before:

While after fixing the N+1 is:

The other field is namespace_settings. The request before is:

While after the fix is:

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Related to #21121 (closed)