Add Codecov coverage reporting

[snabb]

Summary

• add a dedicated GitHub Actions coverage job that uploads reports to Codecov
• use the repository CODECOV_TOKEN secret for the upload step
• add a Codecov badge to the README

Testing

• go test ./...

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️