Add an ability to set the ActorId in BlackBoxBoundedContext

[ndolhii]

Currently, the BlackBoxBoundedContext sets the ActorId on its own and we can't modify it.

But for testing purposes, we may need to emulate commands from different actors.

The way to achieve this is to add an ability to set the desired ActorId to the BlackBoxBoundedContext. From my perspective the API can be the following:

context().receivesCommandFrom(UserId actor, CommandMessage c);