ci: add code coverage job

[jagu-sayan]

Hello @orf :)

Context:

• unit and integration tests are done

Problem:

• no ci to launch this tests and see coverage progress

Solution:

• use Codecov to upload code coverage generated with llvm-cov, see:
  • https://github.com/taiki-e/cargo-llvm-cov?tab=readme-ov-file#github-actions-and-codecov

Integration to Codecov:

• Create an account
• Get CODECOV_TOKEN value
• Create CODECOV_TOKEN repository secret here

You can also add a coverage badge in README.md similar to: