Add coverage stats to CI.

Description

This patch adds a CI pipeline that builds the project with --coverage, runs the unit tests and collects the coverage data with gcovr. A coverage statistic is printed to the console log, which is extracted with a regex. The current gitlab runners don't support SVE, so all coverage statistics for files containing the string "sve" are excluded.

Unfortunately, gcovr's parser is unstable, so the docker image has been fixed to a version that is known to work for us, in this case ubuntu 22.04.

Putting keywords in the the top section is deprecated, so all default settings have been moved into a default section.

DEBIAN_FRONTENT=noninteractive is set, to prevent apt waiting for user input.

Checklist

Please ensure:

• You are familiar with the contribution guidelines
• All new functionality has sufficient test coverage
• All unit tests are passing
• Any C++ code has been formatted with clang-format
• The changelog is up to date
• Implementation statuses are up to date
• Your contribution meets OpenRNG's license terms