EmbeddedModel indexes

Indexes is not created when using embedded models, e.g:
```python

class User(EmbeddedModel):
name: str = Field(index=True)
. . .

class Product(Model):
owner: User
. . .
```

Reproduce the embedded-model example and inspect the code path that builds MongoDB indexes for models and embedded fields. Confirm how indexes are collected, then verify that an index declared on User.name is created when User is embedded in Product.

Written by the indexing model from the issue text.