Enhanced "Aggregate Handler Workflow"

[jeremydmiller]

Using it with a client, and I've seen a couple wrinkles to iron out. This will require a little bit of work on the Marten side too.

1. Sometimes, even frequently, the Aggregate isn't necessary for the "decider" function. So in this case, don't even build out the aggregate. Still enforce all the same concurrency checks, but don't bother for building out the aggregate
2. Make damn sure you don't have to use the Aggregate at all on "create" handler methods that return an IMartenOp. Thought this was done, but still
3. Go ahead and do the optimizations at the Marten level such that an Inline aggregate is never fetched more than once. Opt into the identity map for just that aggregate type on the session

cc @gergelyurbancsik

[jeremydmiller]

Notes

• Use a special signature for message handlers to start it? Works on message handlers, but maybe not so much for HTTP endpoints. Something like Start(command) that returns the events? Does an StartStream using the aggregate type.
• For HTTP endpoints, think about this

[AggregateHandler(typeof(SomeAggregate))]
public static class SomeAggregateHandler
{
    // To start the aggregate. Would be able to follow the same rules for message signatures 
    public static Events Start(command)
    {
         // Naming convention does a new stream
    }

    // Enable Wolverine to use the workflow w/o having to first build the aggregate. 
    // There's not *always* a need to do the validation
    // Might require a Marten FetchForWriting() that doesn't do the aggregation????
    public static IEnumerable<object> Handle(command)
    {

    }

    // IF the aggregate is updated through an Inline projection, make sure you don't
    // double fetch so that the aggregate is used for the projection as well
    public static IEnumerable<object> Handle(command, aggregate)
    {

    }
}

And in HTTP endpoint world:

// TODO -- figure out a better way to do starting a stream?

// MAYBE use a custom IResult for writing the aggregate from a Start???

    

    [Transactional]
    [WolverinePost("/sidekicks/{sidekickId}/rename/{name}")]
    // If Inline, update the Sidekick object built up originally
    // If live, update the Sidekick object with new events before writing to HTTP response
    public static (Sidekick, SidekickRenamed) HandleAsync(
        Guid sidekickId, 
        string name, 
        [Aggregate("sidekickId")] Sidekick sidekick
    )
    {
        var renamed = new SidekickRenamed(sidekickId, name);
        return (sidekick, renamed);
    }