WIP json:api helpers

[moxley]

In response to #12

This is a feasibility study into supporting json:api helpers. It is only a partial solution.

Description

This solution provides a function that returns a %Schema{} struct, which can then be passed to the Operation spec. The struct defines a json:api "document", which is the object representing a json:api request or response body.

The ability to pass a %Schema{} to the Operation spec is also new in this PR. Often, it was necessary to create a schema module that did very little except wrap around another schema that was of interest. The key to making this work is to leverage the "x-struct" attribute in %Schema{}. If this attribute has a value, that tells SchemaResolver to add it to the /components/schemas/* in the Open API document. Using "x-struct" feels like a hack, so I'm eager to hear opinions on a better approach.

Check out OpenApiSpexTest.JsonApiController in this PR for top-level usage.

[mbuhot]

I like the general approach of generating OpenApi schemas by transforming a more natural description of the JSON:API resource.

Also the ability to have inline schemas with titles without needing to define a module looks useful 👍

[moxley]

Replaced with #289