Fix 102: Fix nested constants render issues

[helen-m-lin]

closes #102, closes #156

• added CustomTextWidget to enable custom behavior for constants/ pydantic Literals
  • If const, render input textbox as readonly, and update parent formData to const value using onChange callback
  • Non-constants text fields are unchanged
• update preprocessing schema logic to explicitly add missing type for consts and check that the default is set correctly
• update @rjsf/core to latest
• update unit tests as appropriate