Add SerializedRelatedField for write-by-slug, read-as-nested representation

Description

Currently, SlugRelatedField only returns the slug value (e.g. a PK or custom field).
Nested serializers, on the other hand, expand the related object but require verbose input payloads.

In many APIs, developers want a hybrid behavior:

• Write: accept a simple slug (e.g. "address": 5)
• Read: return the full nested representation via another serializer

This PR introduces SerializedRelatedField, which:

• Behaves like SlugRelatedField on writes
• Expands to a nested serializer on reads (if serializer_class is provided)

Example

from rest_framework import serializers
from myapp.models import Address
from myapp.serializers import AddressSerializer
from rest_framework.fields import SerializedRelatedField

class OrderSerializer(serializers.ModelSerializer):
    address = SerializedRelatedField(
        serializer_class=AddressSerializer,
        queryset=Address.objects.all(),
        lookup_field="pk"
    )

Request

{
  "address": 5
}

Response

{
  "address": {
    "id": 5,
    "province": "Tehran",
    "city": "Tehran",
    "street": "Valiasr",
    "additional_info": "Test info"
  }
}

References

• StackOverflow: How to use SlugRelatedField for incoming request and full serialize output
• DRF docs: Custom relational fields

Testing

Tested locally using django.test.TestCase:

python runtests.py tests/test_relations.py::SerializedRelatedFieldTests

Covers both:

• Writing a related object by slug/PK
• Reading a related object with nested serialization

💡 Note: This feature provides a more convenient and expressive way to handle related fields in DRF, reducing boilerplate code and improving API readability.