Add pluggable request body parsers

### Code of Conduct

- [x] I agree to follow Django's Code of Conduct

### Feature Description

Make request body parsing pluggable.

Benefits

1. Performance, can not use different, more performant libraries like python-multipart or mutipart for parsing forms.
2. Extensibility - Support for msgpack, protobuf, CBOR, and custom formats

Prior

- Django REST Framework: parser_classes on views - proven pattern, widely adopted

### Problem

Django's HttpRequest._load_post_and_files() currently hardcodes support for only two content types:
```
# django/http/request.py:411-455
def _load_post_and_files(self):
if self.content_type == "multipart/form-data":
# MultiPartParser
elif self.content_type == "application/x-www-form-urlencoded":
# QueryDict(self.body)
else:
# Empty QueryDict - no parsing
```

### Request or proposal

proposal

### Additional Details

It is related to https://github.com/django/deps/pull/88. It can be a stepping stone for that dep.

### Implementation Suggestions

Proposed API

Body parser can just be an attribute of the `request`. The user can override that attribute if they want to implement their own parser.