Integrate GitLab code quality reports with Code PushUp issues

[matejchalk]

User story

As a GitLab user, I would like to have some feature equivalent to GitHub annotations, so that during code review I can see new issues in source code which Code PushUp detected.

Research

GitLab CI/CD supports Code Quality reports, which are tightly integrated in the MR user interface.

The Code Quality report format maps nicely to our Issue model. The only non-trivial field is the fingerprint, which serves to uniquely identify the violation. We could take inspiration from eslint-formatter-gitlab here, which hashes the relative file path, rule ID and message.

Acceptance criteria

• the code-pushup.yml template creates a code quality report artifact
• issues are converted to code quality report format with a unique fingerprint
• if detectNewIssues is disabled, no such report is created