test: Setup codecov to block PRs from getting merged if they decrease coverage

[tnevrlka]

https://issues.redhat.com/browse/STONEBLD-632

Implemented gating pull requests if they decrease overall test coverage by more than 1 %.
This threshold is subject to change based on feedback, but should cover most false negatives.
Behavior when removing covered lines is subject to change too. see possible alternatives

codecov/project check should be made mandatory by repo admins if lowering test coverage should actually block the ability to merge the PR.

Now ignoring pkg/github and pkg/gitlab because measuring their test coverage is not desired.

Also split Codecov upload into its own job, so it can be re-ran without re-running the tests. This job should fail in case of an error during upload.

Please see some examples of PRs:
decreased coverage
increased coverage, but overall decrease due to unexpected changes, still passed

See how test coverage can differ even between the same exact code:
coverage decreased even though it should not have changed, still passed
please compare with exact same code in a different PR, but with a different test coverage

[openshift-ci]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[tkdchen]

Just notice this is still in draft.

As the comment just added

[mmorhun]

LGTM. Let's try and see how it goes.