Continuously report on code coverage

[paddy74]

• Code coverage is measured and reported on for the usajobsapi/ directory as part of the standard CI pipeline
• Coverage reports shall be displayed in interface for PRs
• A PR cannot be merged if the code coverage is under 80%
• A code coverage badge in README.md states the coverage percentage of the active branch
  • The code coverage badge indicates if the code coverage is under the minimum amount (80%), such as with a color change
• Coverage reports differentiate between test types (unit, integration, functional) as can be identified by their directories (tests/unit, tests/integration, tests/functional)