Cart micro-service modifications

- [ ] Change aggregate

Currently:
```python
class Cart(Aggregate):
    """Cart Aggregate class."""

    user: int
    entries: EntitySet[CartEntry]
```

Must be:
```python
class Cart(Aggregate):
    """Cart Aggregate class."""

    user: ModelRef[User]
    entries: EntitySet[CartEntry]
```

- [ ] Get Cart items by User UUID