JSONAPI support

**Is your feature request related to a problem? Please describe.**

Workday Peakon currently has a large REST API based on the [JSONAPI specification](https://jsonapi.org). We're in the process of defining an OpenAPI spec for the API and using feTS for typing the client/server interaction. However, OpenAPI does not support [sparse fieldsets](https://jsonapi.org/format/#fetching-sparse-fieldsets) and [inclusion of related resources](https://jsonapi.org/format/#fetching-includes), which results in poor type safety. These features mean that the response type varies depending on the request.

**Describe the solution you'd like**

In a perfect world, feTS would understand the query parameters `fields` and `include` from the JSONAPI spec, and alter the response type correspondingly.

I've drafted a small example of how this could work as a user of the library here: https://github.com/andreas/fets-jsonapi/blob/master/index.ts

It's unclear to me whether this fits in core feTS, or whether it would be more suitable as an extension library or plugin.

**Describe alternatives you've considered**

Accepting poor type safety 😭

**Additional context**

As mentioned, this might be slightly outside the core remit of feTS, but @urigo encouraged me to open an issue here 🙂

Start with the linked fets-jsonapi index.ts example, then read the JSON:API sparse fieldsets and inclusion specifications. Inspect feTS's existing OpenAPI response-typing and extension points to determine whether fields and include belong in core or a plugin. Done means the scope and integration boundary are settled, with corresponding response types for these query parameters.

Written by the indexing model from the issue text.