Allow for recursive data models

[Kircheneer]

Environment

• DiffSync version: latest

Proposed Functionality

Allow for models to have a model of their own type as children.

Use Case

When modelling locations for example, a location of type country might have locations of type city as its children. Example (assuming #222 is implemented):

class Location(DiffSyncModel):
    _modelname = "location"
    _identifiers = ("name", "location_type", "$parent")
    _children = {"location": "location_children"}

    name: str
    location_type: str

    location_children: List['Location'] = []

[jamesharr]

Duplicate #238 has some more example model code that might be useful for initial testing.

The short version is that it looks like it's not far away from working. Where it's broken right now is that children items are processed multiple times.

I don't know the internals well enough to know if this is a change to an iteration method to avoid double processing items, or if it's more of a structural change where the API will need to change to support multiple backends