ci: add a monthly (or on demand) TIOBE reporting workflow

[tonyandrewmeyer]

Once a month, send the latest code quality (including coverage) data to TIOBE for central storage.

[tonyandrewmeyer]

@benhoyt the TIOBE piece ought to be correct (although you'll need to add the secret here, as you did for operator) - I'm not entirely sure whether this is the way you'd best get an XML coverage report for Pebble or not, though, so review around that part would be particularly useful.

[benhoyt]

A few minor comments. I've also added the token to the GitHub Actions secrets.

[benhoyt]

Thanks!