Add downloadable artifacts endpoint

Ref: #324003 (closed)

What does this MR do?

This MR creates a new endpoint which will be used to query downloadable artifacts from the index pipeline page.

API

For consistency purpose the team decided to keep the same schema currently being exposed by PipelineDetailsEntity.

Request	JSON response
/root/coverage-report/-/pipelines/:pipeline_id/downloadable_artifacts.json

Why are we doing this?

Part of the ~performance improvement we are doing on the index pipeline page, we decided to load all artifacts for a given pipeline once a user click on the download button as you can see below 👇

1. First step is to introduce this new endpoint which will be used by the frontend to query the required data.
2. Next step will be to prevent those artifacts from being loaded on the pipeline page. This will be done in a follow up MR.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is not a user facing change.
• [-] 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