Implement custom fields

[Joel-Luca]

Description

• Implement custom field models
• Implement custom field model base manager for custom field annotations
• Implement custom field base serializer for custom field serialization
• Implement custom field viewset
• Improve dummy app

Belongs to PBI TI-2893.

Checklist

The following checklist should help us to stick to our "definition of done":

• Proposed changes include tests.
• Good error handling with useful messages
• Changelog added
• Django migration files have a meaningful name (not 0000_auto_xyz.py nor 0000_alter_xy_model).
• Strings are set and translation catalogs have been updated (i18n-scan or i18n-update) and translations made.
• Readme has been updated if changes interfere with the setup process.

[xenoy]

We need to discus, the commants to see what needs to be changed and what could be done at a future point.

What i would generally discus is the implementation of choice values. I think they don't need to be handled that differently. The main difference to other values is they already exist and are the only valid options. If the identifier (value) of the custom value in combination with the field is made unique and you handle every value as if it would be a choice value. This way you don't need the hack to map the id as you can use value and field as identifier. This would also reduce the amount of entries as if two instance had the same value, regardless if it is a choice field, fhey would point to the same value.