Reworked the generated RegisterAll extension

[Umoxfo]

... to ensure it is AOT-safe

1. IMessengerRegisterAllGenerator.Execute.cs

• Change the IMessenger RegisterAll source-generation to emit public extension methods in the CommunityToolkit.Mvvm.Messaging namespace (RegisterAll(this IMessenger, TRecipient) and RegisterAll<TToken>(this IMessenger, TRecipient, TToken)) instead of producing factory delegates in an CommunityToolkit.Mvvm.Messaging.__Internals namespace.
• Enable nullable instead of null check.
• Update comments.

2. IMessengerExtensions class

• Simplify the runtime fallback/reflection path.
• Update RequiresDynamicCode/RequiresUnreferencedCode annotetion message and reference URLs.
• Update documentation comments.

3. ObservableRecipient class

• Update RequiresDynamicCode/RequiresUnreferencedCode annotetion message and reference URLs.
• Update documentation comments.

4. Unit Tests

• Add new generator test, update expectations, and call sites.

Closes #1123

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
• 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

[Umoxfo]

@dotnet-policy-service agree