GET /api/v1/document/document-fields/{uid}/form-fields/

GET model form fields description to build UI form for EXISTING object:

 - value: any - object field value
 - field_type: str - CharField, IntegerField, SomeSerializerField - i.e. fields from a serializer
 - ui_element: dict - {type: ("input" | "select" | "checkbox" | ...), data_type: ("string", "integer", "date", ...), ...}
 - label: str - field label declared in a serializer field (default NULL)
 - field_name: str - field name declared in a serializer field (default NULL)
 - help_text: str - field help text declared in a serializer field (default NULL)
 - required: bool - whether field is required
 - read_only: bool - whether field is read only
 - allow_null: bool - whether field is may be null
 - default: bool - default (initial) field value for a new object (default NULL)
 - choices: array - choices to select from [{choice_id1: choice_verbose_name1, ....}] (default NULL)