Simplify from_django by relying in Pydantic. Add multiple level relationship support.

[phbernardes]

Hi,

I open this PR/discussion to:

• Add support for multiple level relationships (user -> user.orders -> user.orders.order_items -> user.orders.order_items.order_items_details ...)
• Add support for double underscore aliases (first_name: str = Field(alias="user__first_name")). We can replace it by dot aliases if you prefer (first_name: str = Field(alias="user.first_name"))
• Simplify from_django logic by relying always that possible in Pydantic, this will increase Djantic performance (reducing serialization time).
• Fix ManyToMany relations serialization: ManyToMany relations should access related field by related_name or by *_set and should return a list of instances (from a QuerySet) instead of a instance.
• Harmonize ID/PK usage: instead of returning pk for some fields and id for other, return id for all.
• Calling XSchema.from_django(xs, many=True) should return a list of XSchema instances instead of a list of dicts.
• Include GenericForeignKey in schemas.

Merging this request will cause some breaking changes:

• ManyToMany relations will return a list of instances instead of a instance. The attribute will be called *_set or the Django's field related_name.
• ForeignObjectRel will return a list of dicts with pk instead of id: [{'id': 1}] instead of [{'pk': 1}].
• XSchema.from_django(xs, many=True) will return a list of XSchema instances instead of a list of dicts.

These changes can be seem in test_relations.py and test_queries.py.

I din't separated this PR in several commits as these changes are linked, but I can separate it if it's hard to understand.

Thank you.

[jordaneremieff]

Thanks @phbernardes! I will give this a look on the weekend.

[jordaneremieff]

@phbernardes this looks great, and I think it's okay to merge. Could you add some documentation to demonstrate the usage of the multiple level relationships? Even just a README example would do.

[phbernardes]

@phbernardes this looks great, and I think it's okay to merge. Could you add some documentation to demonstrate the usage of the multiple level relationships? Even just a README example would do.

Of course, please see the example I added in README in 943c1bd
(I'll squash this fixup before merging)