POC: AOT compatible ObservableValidator

[OleRoss]

Closes #1180

This PR serves as a prototype to showcase a working ObservableValidator that is fully AOT compatible (please see the AOT unit tests for that). Additionally, I have added CommunityToolkit.Mvvm.Aot.UnitTests which can be run using e.g. tests/CommunityToolkit.Mvvm.Aot.UnitTests/TestAot.ps1

Right now, there are the following (breaking?) changes:

• All Fallback tests fail (there would be no reflection-heavy fallback anymore)
• Since we do not rely on the Validator logic for discovering attributes, ... there might be differences in the way we discover attributes / doing the exact validation

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• Tested code with current supported SDKs
• New component
  • Pull Request has been submitted to the documentation repository instructions. Link:
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
• 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
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

Other information

Please note that I would not want to merge the generator in its current state. It only serves as a proof-of-concept that the tests can be green with AOT and is almost fully vibe-coded. If there is interest from the maintainers in pushing this forward, I would be willing to put in the time to properly integrate the generator and cleanup the generated code.