Add code coverage to ci

[Aryex]

We currently don't report code coverage on CI. This PR covers setting it up as part of our workflow.

Three parts:

1. Coverage check on every PR. Ensure it covers both standalone and cluster.
2. Add a coverage gate. Each PR should not drop the coverage.
3. (optional) Add a coverage badge to Readme. Update nightly.

We should aim for a 80% coverage rate on both branch and line, however the current coverage rate could be lower than this and thus we should not yet implement a minimum coverage gate.