Add integration with code coverage reporting solution

[m1kola]

In operator-registry we already use codecov so this can be our baseline. But we should be open to consider other reporting solutons keeping in mind that eventually (out of scope for this ticket) we want to have multiple coverage profiles (unit tests, e2e, etc) and should be able to combine them together.

Note: make test-unit already produces coverage report, but it currently is left unused.

Acceptance criteria:

• We should be able to view visual code coverage report
• We should be able to measure diff in code coverage between master and a pull request
• Test job must fail if there is an issue publishing the report to to code coverage solution
• We should have non-blocking checks on PR reporting the diff