Add IObservable<T> extensions for IMessenger

[Sergio0694]

Closes #408

This PR adds observable extensions for IMessenger with the following public API:

namespace CommunityToolkit.Mvvm.Messaging;

partial class IMessengerExtensions
{
    public static IObservable<TMessage> CreateObservable<TMessage>(this IMessenger messenger)
        where TMessage : class;

    public static IObservable<TMessage> CreateObservable<TMessage, TToken>(this IMessenger messenger, TToken token)
        where TMessage : class
        where TToken : IEquatable<TToken>;
}

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