Enable code coverage history

[AArnott]

The ReportGenerator supports producing reports with code coverage graphs. Although ADO doesn't have this built-in, we could generate our own HTML reports that includes this graph as part of our merge coverage job.

To accumulate history, each build could:

1. Download a history artifact from the prior build from that same branch.
2. Contribute a merged coverage file to that directory
3. Remove very old reports to keep the number of files in the artifact to some max number (e.g. 20).
4. Publish the updated artifact.

Storing history as a build artifact would allow history to always be based on past builds from a given branch, even old servicing branches would maintain history so long as we have a prior build for that branch.

[AArnott]

As we'll be publishing our own HTML report, keep in mind coverlet-coverage/coverlet#474 (comment) which explains how to suppress the automatic regeneration of HTML reports by the ADO task.