Allow each DataObject subclass to define its own formfield when autoscaffolded from relation

[GuySartorelli]

Currently, we have the following hard-coded logic for autoscaffolding relations:

• has_one relations are autoscaffolded as one of:
  • DropdownField if there's less than a configurable threshold
  • NumericField if there's more than a configurable threshold
  • FileHandleField (usually UploadField) if it's a file or image
• has_many and many_many relations are autoscaffolded as GridField fields inside a tab.

Some models have specific form fields they should be used with. For example, a Link model might have a specific LinkField for has_one and MultiLinkField for has_many relations.
Currently, adding the form fields for these models often requires first removing the original autoscaffolded field. It would be way better if it just autoscaffolded the correct field from the start.

We should allow each model to instantiate its own form field when being autoscaffolded from a relation.

Acceptance criteria

• The current default logic still applies (i.e. no breaking changes)
• Any model can pre-define its own autoscaffolded form field

[mooror]

This would be a marvelous addition to the DataObject infrastructure 💯