Swith coverage report with ObjectStorage

Part of #211410 (closed)

Step 4 to make code coverage available for everyone.

This feature is behind a feature flag (coverage_report_view). The full feature has been developed on this POC branch.

What does this MR do?

This MR modifies our current logic to fetch our coverage report.

Before this MR	After this MR
Loading the report in memory	Loading the report from ObjectStorage

It implements the necessary logic in order to not alter the behaviour of our current feature:

It includes the following items:

• Adds raw_report to read report from ObjectStorage
• Adds fetch_coverage_for to render coverage only for modified files
• Adds with_code_coverage to fetch pipeline artifact with associated code coverage

Why are we doing this?

Today our Code Coverage feature does not scale properly for large projects containing a lot of code to analyze.

Processing data in memory and parsing big chunk of data have some ~performance implication as explained on this issue.

This is the final step to improve the ~performance of our coverage report feature.

How the feature looks like?

You can visualize how this feature is showing up on the UI on our demo project.

Context

This MR represents a vertical slice of functionality for improving the ~performance of our Code coverate feature. This is slice 4/4.

This work will occur in 4 slices:

1. Create data model - !37969 (merged)
2. Introduce a new uploader - !38221 (merged)
3. Process and persist reports when a pipeline is finished - !39236 (merged)
4. Fetch report from new abstraction - !39721 (merged)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - Changelog to be included with feature flag removal.
• [-] Documentation (if required) - Documentation to be included with feature flag removal.
• 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