Akka.Actor: remove setter from `IWithTimers`

[Aaronontheweb]

Version Information
Version of Akka.NET? v1.5.52
Which Akka.NET Modules? Akka

Describe the bug

Users should not be able to modify IWithTimers

akka.net/src/core/Akka/Actor/Scheduler/IWithTimers.cs

Lines 13 to 20 in 1f7ffa7

	public interface IWithTimers
	{
	/// <summary>
	/// Gets or sets the TimerScheduler. This will be automatically populated by the framework in base constructor.
	/// Implement this as an auto property.
	/// </summary>
	ITimerScheduler Timers { get; set; }
	}

The reason why is we automatically set this in the ActorBase constructor

akka.net/src/core/Akka/Actor/ActorBase.cs

Lines 122 to 131 in 1f7ffa7

	protected ActorBase()
	{
	if (ActorCell.Current == null)
	throw new ActorInitializationException("Do not create actors using 'new', always create them using an ActorContext/System");
	if (this is IWithTimers withTimers)
	withTimers.Timers = new Scheduler.TimerScheduler(Context);
	Context.Become(Receive);
	}

Problems are:

• If user sets this, the ActorBase class is going to reset it, which will cause any timers scheduled in the CTOR to get lost possibly
• VS / any other IDE is going to complain about nullability / et al having this unset Timers property.

Solution

• Force users to set the timer scheduler themselves
• Make it a built-in property of ActorBase and not require users to decorate. Have timers built-in to all actors automatically. There will be a small perf hit from this on the message receiving pipeline but it's not the end of the world.
• Add a Roslyn Analyzer warning telling users not to set it, which is dumb because it means the users are going to get complained at either way.
• In newer versions of .NET, maybe this can be solved with default interface implementations.

Additional Context

Stashing also has this problem!