[FEATURE REQUEST] Support Custom Schema per field

[Noah-Houghton]

Currently, there is no way to override the generated JSON Schema for individual fields. This is problematic when I want to implement highly customized fields, e.g. fields whose selection sets a number of values. It would be useful to allow some amount of customization or override on a per-field basis, so that highly custom implementations can be done without effecting the rest of the form generation.