Add Singleton Pattern Helper to Microsoft.Toolkit

[michael-hawker]

Issue: #2989 (Related to Graph Controls)

PR Type

What kind of change does this PR introduce?

• Feature

What is the new behavior?

Common Helper for using Singleton Pattern based off common established method. Also used by Windows Template Studio.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link:
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

Other information