Add code coverage to the repo

Description

Add code coverage to the phylum-dev/phylum-ci repository for the phylum-ci package. This is both for computing code coverage and enforcing it.

Compute the code coverage automatically in a workflow
- The Test workflow is a likely candidate since it could also be the place to fail the build if coverage drops
- Provide a report as an artifact...even when the build fails
- HINT: use the always() logic in GHA for this
- Fail the build if coverage drops below a set threshold
- Not sure what is normal here...maybe 80% ???
- Might also be okay to just ensure coverage does not decrease
  - HINT: use the actions/checkout action, ref input to check both the main branch and the current ref to get a diff
Expose the ability to compute coverage locally
- Adding to the test(s) tox environments is a good bet here
Consider the pytest-cov package/extension for integrating easily with the existing pytest package

[ ] Code coverage is computed automatically in CI
[ ] Code coverage is available to be computed locally
[ ] Code coverage is enforced in PRs via failing required checks for minimum coverage threshold
[ ] Documentation is updated