Returns the fields required to submit an offer enrollment for the given offer_id.
The field schema varies by offer — pass the offer_id from the offers endpoint and
use the returned fields to construct the POST /form request body.

Conditional Field Schemas

dependent_key / dependent_value (conditional fields)

Used by any offer whose form has conditional fields, such as EV charger installation.
A field with dependent_key and dependent_value is only shown (and required) when
the referenced sibling field equals the given value.

someField:
  dependent_key: otherField     # watch this field
  dependent_value: someValue    # show me when it equals this

cascading_group (chained dropdowns)

A field with cascading_group participates in a multi-level chained dropdown.
Each level filters the options of the next level based on the selected value.

Important difference from dependent_key:

• dependent_key hides/shows a field based on a binary condition (show or hide).
• cascading_group narrows the allowed options of a field based on the parent
  selection. The field is always visible; its option list changes dynamically.

Vehicle selection cascade:
Three chained levels: vehicleMake (level 1) → vehicleModel (level 2) → vehicleYear (level 3).
Makes and models are sorted alphabetically; years are sorted newest-first.