Django-api-forms

- **Fixed**: Removed validation of non-required fields if they are not present in the request

- **Fixed**: Fixed "weird" behaviour with missing `clean_data` values if using `ListField`

This release has been inspired by [Problem Details for HTTP APIs - RFC7807](https://tools.ietf.org/html/rfc7807) and
blog post [Structuring validation errors in REST APIs](https://medium.com/engineering-brainly/structuring-validation-errors-in-rest-apis-40c15fbb7bc3)
written by [k3nn7](https://github.com/k3nn7).

The main idea has been to simplify validation process on the client side by flattening errors output. To achieve such
a goal, the whole validation process has been rewritten (and luckily for us, much simplified).

- **Changed**: Positional validation errors for lists
- **Changed**: `ImageField` and `FileField` requires [Data URI](https://datatracker.ietf.org/doc/html/rfc2397)
(issue [Raise ValidationError in invalid Data URI by default](https://github.com/Sibyx/django_api_forms/issues/22))
- **Removed**: `Form.fill()` method replaced by `Form.populate()`
- **Removed**: `fill_` methods replaced by population strategies

- **Changed**: Raw base64 payload in `FileField` and `ImageField` fires `DeprecationWarning`. Use Data URI instead.

- **Added**: Introduced `mapping`
- **Added**: Override strategies using `field_type_strategy` and `field_strategy`

- **Fixed**: `DictionaryField` was unable to raise validation errors for keys