Generate partial method or call method optionally.

[Kation]

Closes #1178
This pull request adds optional for ObservablePropertyAttribute to indicates whether to generate changing/changed method and call changing/changed method.

[ObservableProperty(GenerateOnChanging = false, GenerateOnChanged = false, CallOnChanging = false, CallOnChanged = false)]
private partial string _field;

Indicates not generate changing/changed method still call them when setting property.
Thereforce we can make a protected method.

[ObservableProperty(GenerateOnChanged = false)]
private partial string _field;

protected virtual void OnFieldChanged(string? oldValue, string newValue) { }
protected virtual void OnFieldChanged(string value) { }

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

Documentation have not update.