WIP: Add basic fuzz targets

What does this MR do?

This MR adds basic fuzzing to gitlab-runner, currently only fuzzing the YAML parsing.

Why was this MR needed?

To dogfood upcoming fuzzing features and increase the stability and security of gitlab-runner.

What's the best way to test this MR?

What are the relevant issue numbers?

Other Requirements

• Make sure https://gitlab.com/gitlab-org/security-products/analyzers/gitlab-cov-fuzz-src/-/issues/1 is addressed