Add expected version to EventStore interface #10

[watfordsuzy]

This PR is merely to discuss issue #10 and I don't feel confident enough in the foundational API shapes to suggest otherwise.

Trying to adapt Emmett to some problems I found myself smuggling expected versions within event metadata rather than the API itself, while also having the next expected version returned through the API.

[oskardudycz]

@watfordsuzy, thank you a lot for the PR and for the issue. Indeed, the lack of optimistic concurrency is a big thing. I didn't add that initially without the intention to hide that part or ignore it. The first release was focused on the business logic; now it's time to do the application layer correctly.

I'm planning to do it more or less like in the solution in my self-paced kit: https://github.com/oskardudycz/EventSourcing.NodeJS/blob/2d417bd2e650c16b4af68b6a752bd684ce0a8fc0/workshops/introduction_to_event_sourcing/src/solved/07_optimistic_concurrency_eventstoredb/tools/eventStore.ts#L99

I'm going to pull your changes in and restructure them a bit. Thank you a lot for the contribution and double-checking if I didn't miss it!