Added Api Specification for integration testing

[oskardudycz]

In Emmett, you get lightweight abstractions like event store, command handlers, etc. Event Sourcing gives repeatable patterns for handling business logic:

1. Read events and build the state.
2. Take that state and a command and run business logic.
3. Store new event(s) as a result.

That's super easy to test, especially in the BDD style:

1. Given a set of events.
2. When the command is run.
3. Check the result events.

Now, I had a thought: why not apply the same for API tests? Such tests could even be written in the Hexagonal Architecture style by replacing the external dependencies with some in-memory implementation or just going fully end-to-end.

The benefit of that is that you can test the whole pipeline (middleware, auth, etc.), even Open Telemetry traces (like Martin Thwaites showed in his talks).

This PR introduces the first version of such tests.