Protect source files already at 100% code coverage - add "100PercentClub" build pipeline check

[stuartpa]

The modern CLI uses a dependency injection architecture which makes it easier for most source files to get to 100% unit test code coverage. To keep this 100% code coverage culture moving forward for new source files, it would be good to be able to annotate source files that have once hit 100% unit test coverage, to not be able to drop below 100% again (a build pipeline break will prevent PRs merging that regress a source file that has previously been at 100% code coverage)