Fix 139: Enable validation for discriminator keyword

[helen-m-lin]

closes #139

1. Enabled discriminator validation in the JSON Schema validator.
   • This ensures that validation is only done against selected option rather than all oneOf options (resolves the repeated errors for all fields).
   • Added pre-processing to conform to Ajv validator requirements (docs).
2. Enabled liveOmit and omitExtraData in the RJSF props (docs).
   • This ensures that extra formData is not preserved when the selected option is changed (resolves the repeated must NOT have additional properties errors).
3. Added unit tests as appropriate.