Publish coverage results in main branch into a report suitable for Gitlab

It seems that Gitlab does have readymade visualisation for main branch coverage changes. As we already have this test running after !133 (merged), we could tune the CI script a bit more to get the correct format and start seeing the report. The report is obtained somehow like this

https://docs.gitlab.com/ee/ci/yaml/artifacts_reports.html#artifactsreportscoverage_report

and then we need to get the data to Cobertura format somehow. As the tools we are currently using are gcov and lcov, then this one no top of them might be the easiest

https://pypi.org/project/lcov-cobertura/

Another option would be to use gcovr.

The results should start collecting here then.

https://forge.3gpp.org/rep/ivas-codec-pc/ivas-codec/-/graphs/main/charts