Support ActorReference

[artursouza]

Describe the proposal

.Net SDK supports ActorReference class as a convenience for users to return the ActorType-ActorId tuple. This class should also be implemented in Python SDK.

https://github.com/dapr/dotnet-sdk/blob/master/src/Dapr.Actors/ActorReference.cs

Also, update the actor_sdk E2E test by adding a invocation validating this use case cross SDKs.

[artursouza]

ActorReference is a class that can be serialized by user provided serialization and it might not be compatible with every serializer, specially when cross SDK compatibility is required. I propose we pause this until further discussion about cross SDK serialization.