Context

this pr is fixing : #1501

it allow to do

from docarray import DocVec, BaseDoc, DocList
from docarray.documents.image import ImageDoc

class Request(BaseDoc):
    images: DocList[ImageDoc]
        
images = DocList[ImageDoc]([ImageDoc(url=f'http://url.com/{i}.png') for i in range(10)])
request = Request(images=images)
request.json()

without having an error

what this pr exactly do:

• when calling json it manually deal with the nested DocList, otherwise Pydantic does some weird logic because it think it is a List (which is it)
• Custom validation on DocList. Now if the input is not a DocList but a list of dict lets say, it will try to validate every member of the iterable with the doc_type and then create a DocList out of it