[Feature] Support for providing custom field types/Q_TYPES in template.json

[Udayraj123]

Is your feature request related to a problem? Please describe.
Currently we need to change the code in the constants.py file and modify BUILTIN_FIELD_TYPES to support any kind of custom field types in a custom OMR sheet. This needs to be handled via template.json instead.

Describe the solution you'd like
Introduce a key customFieldTypes that automatically gets merged with BUILTIN_FIELD_TYPES and is available to use in the fieldBlocks. This eliminates the need for changing the BUILTIN_FIELD_TYPES.

Describe alternatives you've considered
NA

Additional context
NA