Enable coverage information generation and tracking

This MR enables the generation of code coverage reports for the source code. More specifically, it introduces the following changes:

- Adds the `gkfs-code-coverage.cmake` CMake script under `{PROJECT_ROOT}/CMake/` and includes it in the project's main `CMakeLists.txt`. This script defines the `GKFS_ENABLE_CODE_COVERAGE` option to control whether code coverage should be enabled when building, as well as the `target_code_coverage()` function to ensure that the appropriate flags are added to a CMake target.
- Adds `gcovr` to GekkoFS docker build image.
- Adds a `coverage.sh` script under `{PROJECT_ROOT}/scripts/ci` that provides both a `capture` and a `merge` work modes. This script abstracts away the direct usage of `gcovr` and allows the appropriate generation of coverage reports when running tests **in parallel** in the CI. The script relies on a `.coverage-exclusions` file (also under `{PROJECT_ROOT}/scripts/ci`) to control which paths should be excluded from coverage reporting (e.g. `/usr/include/.*`).
- Adds a new `report` stage in the CI pipeline that upon completion generates artifacts both for a Cobertura XML and a HTML code coverage reports.

See merge request hpc/gekkofs!77