Feature-request: Allow to use both ObjectIDs and DBRefs as links (To ease migration from MongoEngine)

[cdelacruzpinto]

Suppose we have the following structure:
`
class Door(Document):
height: int = 2
width: int = 1

class House(Document):
name: str
door: Optional[Link[Door]]
`

Currently, a link is only resolved automatically by Beanie if in Mongo if it's represented in the actual database like this:

door: DBRef('Door', '65e97bc6e6b5d4fa42bc2581')

Currently, if I replace door: DBRef('Door', '65e97bc6e6b5d4fa42bc2581') with door: ObjectId("65e97bc6e6b5d4fa42bc2581" in the database, beanie also infers the collection name, because a "print(house1.dict())" yields this:

{'id': '65e97bd7a4da61784f74bfbf', 'name': 'Casa carlos', 'door': {'id': '65e97bd7a4da61784f74bfbf', 'collection': 'Door'}}, But won't resolve the actual content of the door object which is

{'id': '65e97bd7a4da61784f74bfbf', 'name': 'Casa carlos', 'door': {'id': '65e97bd7a4da61784f74bfbe', 'height': 1, 'width': 2}}

In a nutshell, my feature request is:

Support links defined in both ways:

door: ObjectId("65e97bc6e6b5d4fa42bc2581")
and DBRef('Door', '65e97bc6e6b5d4fa42bc2581')

This makes migrating a service from MongoEngine to Beanie easier, as MongoEngine represents links in both ways.