Fresh take on Virtual Actors - allowing "get user for some user defined id"

[ktoso]

We should have a fresh take on #288 that is based on distributed actors.

Virtual actors are a type of "an actor that is always there" and are allocated on demand by some user id.

[ktoso]

It'd be a take on Orleans style virtual actors; implementation wise we can base it on Akka's sharding for the most part I believe.

Prior art:

• https://doc.akka.io/docs/akka/current/typed/cluster-sharding.html akka's cluster sharding is effectively the same feature, but not packaged up in the same API
• https://dotnet.github.io/orleans/docs/grains/grain_placement.html orleans primary abstraction is virtual actors

[akbashev]

Even from my little experience already facing some limitations, where it's good to have actor by id. So would like this feature 👍