Feature: Add support for CustomFields

[tannermeade]

DataOrigins have a use case where the schema fields have special representations that need to be identified by the DataOrigin to upload and save the data correctly.

This will be defined by extending SchemaFields (and maybe overriding validation methods).
DataOrigin uses the SchemaField runtime type to handle the schema creation and data upload.
Use cases:

• Appwrite’s $id can be defined and uploaded