Add IgnorePatterns for flexible prebuilt baselines

[dagood]

This change adds support for IgnorePatterns to the prebuilt usage baseline file. This means a repo on arcade-powered source-build can have this content at eng/SourceBuildPrebuiltBaseline.xml to ignore all prebuilt usage when onboarding:

<UsageData>
  <IgnorePatterns>
    <UsagePattern IdentityGlob="*/*" />
  </IgnorePatterns>
</UsageData>

Then, usage can be gradually decreased and the UsagePatterns can be tweaked to be more specific to the repo. The eventual goal is to allow necessary prebuilts (e.g. cycles in the dependency graph), but make unexpected new prebuilts show up as PR validation failures. This is for dotnet/source-build#1715

It supports regex via IdentityRegex for even more flexibility if needed.

The ignored usage still shows up in the more detailed reports for analysis, but they don't cause an error. (Or cause a warning--because warnings are treated as errors in Arcade.)

I'm also planning to port this to dotnet/source-build for more sustainability there: dotnet/source-build#1726.

This PR also adds a simple test project for a few scenarios I wanted to verify. (This is the test project I tried to add in #6121, but I'd backed it out when I didn't want to spend the time to figure out why it was failing in CI.)

[dagood]

@markwilkie Who from Arcade would be interested in looking at this? I think the potentially interesting/disruptive thing is the added test project. (The source-build task DLL is only used very specifically when arcade-powered source-build is enabled.)

[dagood]

Mark mentioned in a chat that he's alright with our own reviews here, since the risk of a break here is pretty much just for us. 😄 Merging.