Add new endpoint for pipeline badge counter

What does this MR do?

Step 2: to make Junit Test Report faster as explained on #37725 (closed)

This MR is the second step of performance improvement we are doing to make our new feature JUnit Test Report more resilient.

It introduces 2 concepts :

• A new endpoint which is responsible for rendering our test report badge counter to reduce the amount of data parse and render by the server.
• A new method caching the result of this number at the pipeline level.

This MR introduces new code behind our feature flag junit_pipeline_view and will not be using by any code path yet.

We will follow up and use this new endpoint when loading the badge counter asynchronously after this MR !24522 (merged)

Screenshots

This MR is modifying this part as you can see on the screenshot below:

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