Add ability to cap the number total number of active actors (by type)

[dale-personal]

Add a MaxActiveActorCount setting similar to other settings in ActorRuntimeOptions that would cap the number of active actors of a given type.

A use case might be to support authenticated and unauthenticated actors within a system. but capping the number of unauthenticated actors in order to have a more deterministic calculation of resource requirements with a knowable number of authenticated users + a capped (otherwise unknowable) number of unauthenticated users.

Once the max is hit, ActorProxy.Create() would throw an appropriate exception.