Add support for v2 enum choice lists

[doneill]

Add support for EarthRanger server v2 jsonform schema transformations that replace oneOf arrays where each item is in the form of

{ "const": ..., "title": ... }

with a semantically equivalent enum array. The original oneOf schema is retained separately, as the title values on each const entry are required to render the form field options correctly.

Example:

"oneOf": [
  { "const": "uuid-1", "title": "Source 1" },
  { "const": "uuid-2", "title": "Source 2" }.
  ...
]

Is transformed into:

"enum": ["uuid-1", "uuid-2", ...]

Action:

• Accept enum & x-enumExtra as a valid CHOICE_LIST structure
• Handle no options with empty enums

Expectation:

• validateV2Schema accepts and handles enum as choice list for display in React Native app implementation