Allow joins in record tables

[camdendotlol]

From NBU:

Develop a generalized solution that makes it possible to see the document associated with a person record when looking a table of people.

In other words, the feature request is to allow joins in the column selector.

[camdendotlol]

(this comment is a WIP)

Broadly speaking, there are two things we have to do:

• communicate to the frontend which nested fields are available so we can display them in the field picker
• offer a way for the backend to request, and be provided with, specific nested fields

Field display

Currently, when loading a record list, CD fetches information about the model from the project_models endpoint. This is how user-defined fields are populated in the list:

We would want to expand the response from that endpoint to include fields from related models, then add handling to the controllers to support nested fields that are included in the request.

Field querying

So I know for sure that we would want to override the index methods for each model. It's possible that we would want to implement this in resource-api instead of directly in CDC. https://github.com/performant-software/resource-api/blob/ef159655504a6011a453c78968036e9c2f58890f/app/controllers/api/resource_controller.rb#L44

We might also want a new Nestable concern on CDC where models can set which fields are allowed to be displayed via another model.

The index method would need to take care of preloads, sorting, and searching in addition to just returning the data.