Handling references to external models by `id`

Hi,

I'm working on a FastAPI application with ODMantic and it's fantastic. However, I'm a bit confused by how references to external models are handled. Maybe I did it all wrong, but I came up with the same solution than https://github.com/art049/odmantic/issues/127#issuecomment-887339892 in order to avoid a validation error

Let's say that I'm trying to model a project and its builds:

```
class Build(Model):
project: Project = Reference()
name: str
date: datetime
success: bool
```

I must use an additional model:

```
class BuildInApi(Model):
project_id: str
name: str
date: datetime
success: bool
```

Or `project_id: Optional[ObjectId]` but it's the same.

And then to:

1. Find the related project.
2. Map the fields between the two models

As in this example endpoint:

```
@router.put("/builds/", response_model=Build, tags=["builds"])
async def create_build(build_in_api: BuildInApi, request: Request):
project = await request.app.engine.find_one(Project, Project.id == ObjectId(build_in_api.project_id))
build = Build(project=project,
name=build_in_api.name,
date=build_in_api.date,
success=build_in_api.success,
payload=build_in_api.payload)
await request.app.engine.save(build)
return build
```

This is cumbersome and errorprone so I'm sure I am missing something.

The issue provides Build and BuildInApi models plus a FastAPI create_build endpoint; start by reproducing that reference-by-id flow and reviewing ODMantic's reference handling. Done means the external model reference has a clearly supported workflow without the extra mapping model, with validation and persistence behavior verified.

Written by the indexing model from the issue text.