extra_kwargs for nested fields

[BurkovBA]

I suggest the following extra_kwargs syntax for constructing fields in EmbeddedDocumentSerializer:

clas PapaSerializer(DocumentSerializer):
    class Meta:
        model = Papa
        extra_kwargs={
            'kid.foo': {allow_blank: True}
        }

• EmbeddedDocuments
• NestedEmbedded, BottomEmbedded
• NestedReferences
• Compound fields
• EmbeddedDocumentListField
• test validate_methods()
• validators
• source fields - model field name might not be the same as serializer field name
• uniqueness extra_kwargs