WIP: Add Coverlet Coverage to builds

[SeanKilleen]

A continuation of #4286, which got stale. Using lessons from my recent(ish) blog post.

Work Involved

(I think -- still feeling my way around the project in this way)

• Add coverlet.collector to the codebase
• Add TRX logging output to all places using dotnet test
• Combine coverage and surface test reports for each test run
• Add Coverlet add-in to Cake script
• Add Coverlet dotnet global tool
• Utilize coverlet CLI tool or cake task in cake-based build items

[SeanKilleen]

@stevenaw FYI, I restarted things here. Rough weekend, didn't get nearly as much time as I thought I would. But at least it's no longer stale, so, small win? 😅

Next up, I'll look through the cake file to understand where we're outputting our test assemblies and will figure out how to run coverlet on them (since we're not using dotnet test which makes sense, I'll probably look into how to use the Cake CLI add-in to do what we need to do.