Change command result to return event stream id from command instead of result

[jonr-clever]

Change command result for command processor to return event stream from command instead of result. This allows usages of derived instances of EventStreamId from the returned Command result.

E.g. i can outside the ExecuteAsync call use the returned stream id with polymorphism to do the following code:

if (eventStreamId is RfidCardId rfid)
{
    return rfid.Id;
}

given my class RfidCardId:

public class RfidCardId : EventStreamId
{
    public string Id { get; }
    // rest of implementation
}