Add Code Coverage to PR Checks

[ericspod]

Is your feature request related to a problem? Please describe.
Codecov and others will add a report on the coverage metrics for PRs. This would be helpful for us when assessing the coverage of tests for new features and enforce the need to improve coverage and not let it slide.

Describe the solution you'd like
A PR comment report of the coverage delta for the proposed changes, like this.

Describe alternatives you've considered
Manual code coverage checking.

Additional context
We use Codecov now for tracking coverage but adding the PR message would help.

[chrislevn]

Hi @ericspod, I'm new to MONAI and would love to contribute. I have a draft PR here. Will improve it upon your feedback. Thanks.