Support filtering with `relation__in` using model instances

Is your feature request related to a problem? Please describe.

In Django ORM, we can efficiently filter related objects using relation__in syntax, for example

Post.objects.filter(author__in=[user1, user2])

However, Tortoise ORM only supports relation_id__in, requiring us to extract IDs manually before filtering. This makes queries more verbose and less intuitive, especially when working with object instances rather than IDs.

Describe the solution you'd like

Add support for relation__in filtering in Tortoise ORM, allowing direct filtering using model instances similar to Django's implementation. For example:

await Post.filter(author__in=[user1, user2])  # Currently not supported
# Instead of current approach:
await Post.filter(author_id__in=[user1.id, user2.id])

Additional context

This feature would improve compatibility with Django ORM patterns.

Start at the Post.filter entry point and trace how the existing relation_id__in syntax is handled. Compare the desired relation__in examples with current filtering behavior, then add coverage showing that model instances can be passed directly and that existing ID-based filtering remains supported.