[8.x] Add support for dynamic 'for' methods on factories with existing model instances

[bakerkretzmar]

Adds support for attaching existing model instances using dynamic forModel() syntax. Follow-up to #35494 and #35541.

For example, to attach a User model via an author() relationship that uses the author_id column:

$posts = Post::factory(10)
    ->forAuthor($user = User::factory()->create())
    ->create();

See #35494 (comment).

[bakerkretzmar]

Never mind, I forgot that the dynamic methods expect an array, not a factory. We talked about adding this before and decided it would be confusing. I think it makes more sense to just specify the relationship explicitly with something like ->for($user = User::factory()->create(), 'author').