Invest in improvements to the SDK repo

[marcpopMSFT]

Copied and separating from #42167

• Cleanup of old and new issues, test failures, and source
  • Invest in improving existing tests
    • https://github.com/dotnet/sdk/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22Known%20Build%20Error%22
    • https://github.com/dotnet/sdk/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22Test%20Debt%22
    • After we do a pass over the ones I've linked, we should consider just deleting all remaining skipped tests...
  • Identify key customers bugs that need investigation and resolution
  • Identify code for cleanup, refactoring, commenting, and documentation
  • Identify code duplication that can be simplified (shared source files)
• Improvements to the SDK repo infrastructure
  • Combine redist and redist-installer
  • Switch tests to use the installer output and reduce the overlaysdk impact
  • Investigate and improve performance of dev, PR, and CI builds
  • Investigate onboarding additional tests to helix (templating, format)
  • Investigate improvements to the templating tests that are easily broken by live template changes
• Enable Nullable for the SDK repo and gradually enable for various projects
• Enable package source mapping for the repo and tests