Dynamic code coverage percentage

[tim-schilling]

#1558 reintroduced listing the code coverage percentage, but as a static value. Ideally we'd display the actual code coverage percentage from the tests that run on the main branch. With using GitHub actions to run coverage, then uploading the html report as an artifact, we need to store the percentage somewhere for the badge to reference.

This blog post goes through a process in which a gist would be edited to include the code coverage percentage, then the badge would reference the gist's JSON data and render out the percentage. It's admittedly a bit hacky, but it doesn't rely on an entirely 3rd party as we'd be using GitHub's gists as a storage mechanism. It also requires a gist to be created which I'm not sure if we can do with the jazzband org or if we'd need one of us to do it on our personal accounts. I believe it's the latter as I can find the option in an org that I'm an admin of.

@jannisleidel if we go with this approach, we'll need your help to add a repository setting containing a key to create gists under whoever's account that we create the gist with.

[tim-schilling]

Closed in favor of #1904