Move SAST reports logic for Pipelines to the backend

The pipeline reports currently use the generated artifacts to generate SAST reports on the frontend. We should move this logic into the backend, and provide final results to the frontend, like junit reports do.

This will improve performances of page load and will allow more flexibility.

Proposal

Create a new API to deliver SAST reports content to frontend, using a ready to use and common json entity. Such entities already exist as part of the Group Security Dashboard and should be reused to provide the same structure everywhere and allow frontend to reuse standard components.

This API should then be consumed by the frontend and the older artifacts code should be moved behind a feature flag so we can revert back to using it if we need to.