Added source generator for actor type registration

[WhitWaldo]

Description

Using the .NET SDK today, one must register each actor type with dependency injection like so:

services.AddActors(options => {
  options.Actors.RegisterActor<MyActorA>();
  options.Actors.RegisterActor<MyActorB>();
  options.Actors.RegisterActor<MyActorC>();
  options.Actors.RegisterActor<MyActorD>();
});

As one uses an increasing number of actor types, this can get cumbersome to deal with maintaining this list of types. That's where this source generator comes into play. If all the actors are in the same project, the developer need only do the following:

services.RegisterAllActors();

And if the developer is interested in also including actor types from any assemblies referenced by the project as well, they need only indicate as much, as follows:

services.RegisterAllActors(true);

Issue reference

We strive to have all PR being opened based on an issue, where the problem or feature have been discussed prior to implementation.

Please reference the issue this PR will close: N/A

This was the result of a conversation between @philliphoff and myself recently.

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Extended the documentation