Add extension point for implementing custom event sourced entity api

[patriknw]

Such as an FSM style API.

Probably corresponding to EventSourcedEntityRouter a way to implement these two methods in user code:

def handleEvent(state: S, event: Any): S

def handleCommand(commandName: String, state: S, command: Any, context: CommandContext): EventSourcedEntity.Effect[_]

See spike from @jroper at https://github.com/jroper/kalix-fsm-java

[johanandren]

We discussed this and after some meetings with the customer it seems very unclear if this is actually something they need. I'll close and we can revisit.

[jroper]

It is definitely something they need, and I believe they are using it already - that is to say, they are implementing the internal router API that we provide.

[johanandren]

We'll do a meeting with them next week and make sure we understand what they need.