add code coverage badge to github front page - facilitates visual certainty in unit test code coverage

[maxkazmsft]

This can be done a number of ways - start with ADO and try to do it through there. If not possible (badge needs to have unit test code coverage percentage) then look at scripting this up as a build job (add to tests/cicd/main_build.yml) where the job will generate a badge and upload it to public WASB which is referenced in the main README.md (same way current build badges are referenced).