Add codecov as reusable GHA workflow

[ewu63]

Description of feature

Reusable workflows have to be jobs, not steps. So, the tricky part is to pass in the coverage file from the testing job to the coverage processing/uploading job. This can be done via the artifact upload/download approach, see for example here.

[stale]

Is this still relevant? If so, what is blocking it? Is there anything you can do to help move it forward?

This issue has been automatically marked as stale because it has not had recent activity.

[ewu63]

Given that this exists, we can probably close the issue and just use this as a step during testing.