ci: Upload coverage to codecov

[auguste-probabl]

Closes #1840

[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 ☂️

[auguste-probabl]

To really test this, the PR needs to be merged to main; I need to test this in a copy of the repo.

[thomass-dev]

In that case, if codecov is in charge of displaying the coverage, please remove the .github/workflows/pr-display-code-coverage.yml workflow.

[thomass-dev]

Note: please ensure that the coverage of each package (in our mono-repo/multi-package environment) is well managed/identified.

[auguste-probabl]

@thomass-dev I did some experimenting in a copy of the repo, because I need the Codecov step to have run on main to get the diff for a PR. Here is what I did in main: auguste-probabl/skore-copy@4091b54...main

And then I opened a PR: auguste-probabl/skore-copy#3

But it's not necessarily iso with our current setup, with the segmentation between maintainers and external contributors, etc.

Would you mind reviewing what I did on my copy here? Do you think this would work for us? And also we'll have to get a token from Codecov.