Support for testing 'forward'

[raboof]

Part of #372

For both ValueEntity and EventSourcedEntity

[johanandren]

I think that is already possible with the com.akkaserverless.scalasdk.testkit.impl.TestKitServiceCallFactory so just needs to be wired up in the testkit contexts and cast in the result classes.

[johanandren]

Covered by ValueEntityResult.forwardedTo and EventSourcedResult.forwardedTo