Allow derive models from pydantic models

[erny]

See #60. @art049: I create this draft PR to start discussing how this could be done. Thanks in advance.

[art049]

Thanks for the amazing initiative @erny, i think we can maybe try to properly define the specs first and then we can check how it would be feasible.
For sustainability and isolation from pydantic internals, I think we should avoid to pass a custom Field descriptor (which actually generates a FieldInfo object) while building the initial pydantic models. This was a decision I took during the early ODMantic stage and I think it will really help to support future changes that might happen in pydantic itself.

From the example you added in the issue, I think we can try to build a use case taking in account some more customization. For example:

• we want name to be a primary field: equivalent to a regular definition with Field(primary_field=True).
• the founded field should be internally named founded_year in the DB documents: equivalent to a regular definition with Field(key_name="founded_year").
• we want to be able to provide the ODMantic configuration object that will customize the collection name to "book_publishers" (currently, it's not really containing critical data but the future indexes will be defined there for example).

We keep the same business Model definition:

from pydantic import BaseModel, Field

class Publisher(BaseModel):
    name: str
    founded: int = Field(ge=1440)
    location: Optional[str] = None

The persistence model creation would look like this:

from odmantic import AIOEngine, Model, Field
from schemas import Publisher

class PublisherDB(Model, Publisher):
    class Config:
        collection = "book_publishers"
        fields = {
            "name": Field(primary_field=True),
            "founded": Field(key_name="founded_year")
        }

To avoid duplicating model configuration, everything related to pure ODMantic fields configuration would be stored in the fields config property.
As well, the resulting descriptors should take into account the ones from the base class. In this case, the resulting "founded" descriptor should contain both the ge=1440 information as well as the key_name information.

Finally, since we use the inheritance approach, we won't be able to have fewer fields in the DB model than in the Domain one. The opposite should be possible nevertheless and it would allow to add some more database only fields in the PublisherDB definition.

I hope this spec is filling the need you have and i'm obviously opened for some refinement 😉