Introduce code quality metrics to MRs

As previously discussed in the speed up CI MR, managed to make progress from the prototype to make clippy warnings show up in MRs to production ready web service. Decided on the web service as it's fastest to set up compared to installing binaries or pulling Docker images.

Am happy to have it code reviewed first, address issues and then create a first release. It's pretty straightforward as it's using an existing crate that can parse compiler (including clippy) output. https://gitlab.com/dlalic/gitlab-clippy Hope it's documented well enough so the "hit by a bus" is not a factor. MR is in Draft as curl installing needs to go into the image if this is accepted.

Demo is at the same place > see "Code quality degraded on 1 point".