Add reactive-cache to pipeline config variables

What does this MR do?

Calculating config variables is a heavy operation, and we don't want to run a long synchronous operation for every request. So with this, we only call the operation synchronously once, then it will read from the cache until the expiration., the endpoint will return no content if the cache is empty, then the frontend will poll it.

This branch is based on the branch of this MR: !47189 (merged).

• Related to #30101 (closed)
• Implementation: !43392 (merged)
• Frontend implementation to see what is it about: !44120 (merged)

Screenshots

predefined_variables_performance

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