Continuous integration: add integration with Codecov

[priethor]

What

This PR improves the SCF continuous integration by integrating with Codecov.

Why

As a fork, SCF currently has a low code coverage:

• We should ensure that new features do get coverage
• Ideally, as we add coverage to legacy code, it will help us ensure legacy code doesn't break and understand future backports

How

By generating a PHP coverage report during the PHPUnit action, and uploading the results to Codecov

[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 ☂️

[cbravobernal]

Let's see. 🚀

[github-actions]

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