Initial implementation to support bulk extension

[bjornharrtell]

This is an initial spike to implement #215.

[bjornharrtell]

@joukevandermaas do you think something like this could be a reasonable approach to support the bulk extension?

[joukevandermaas]

@bjornharrtell looks like a good approach.

It's not very clear from the examples on the page you linked, but it seems that you should be able to post/patch/delete resources of different types in a single request?

That doesn't seem feasible with this solution, because the enumerable has a fixed type in the action method's parameter. Perhaps we could create a type like this:

class JsonApiType
{
    string Type { get; }
    T ConvertTo<T>();
}

Then force action methods with BulkExtRouteAttributeAttribute to take IEnumerable<JsonApiType>? Or perhaps we don't force it, but document that if you want to support multiple types, you'd do it this way?

[bjornharrtell]

I don't think it makes sense to support different types, as the bulk extension works explicitly on a specific resource collection?

Unrelated, I note that there is a requirement for extension negotiation described at http://springbot.github.io/json-api/extensions/#extension-negotiation.

[bjornharrtell]

Seems I jumped the gun here and that heterogeneous resource collections are indeed supported by JSON-API (although that's a bit of a surprise to me..), at least that's my interpretation of the discussion at json-api/json-api#278. However if Saule currently assumes homogenous resource collections (which I think it does?) I think it's reasonable to have the same limitation here.

[bjornharrtell]

And yet another realization is that this and other extensions have been deprecated (https://jsonapi.org/extensions/#prior-extensions). However, it still does seem quite useful anyway and it doesn't look like the upcoming JSON API 1.1 will include what seemed to be the intended replacement (Operations, json-api/json-api#1254).