Add public API mutation testing CI

[tob-scott-a]

This sets up mutation testing in GitHub Actions.

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider will also need to be ready for review and merge by the time this is merged. Also, make sure to update the list of algorithms in the continuous benchmarking files: .github/workflows/kem-bench.yml and sig-bench.yml)

These changes were generated by Codex-5.5-Cyber as part of the Patch the Planet program.

[coveralls]

Coverage is 82.301% — tob-scott-a:deliverable/ci-mutation-testing into open-quantum-safe:main. No base build found for open-quantum-safe:main.