Add ability to pass block to factory attributes

The recommended way to override attributes of a factory is something like this

```crystal
CommentFactory.create &.post_id(new_post.id).text(sequence("blah"))
```

Now, if `CommentFactory` sets this `post_id` in it's initialize with a call to a `PostFactory` that will end up with two posts made which is not intended and can add up to a lot of random records being created in large test suites.

I believe this is why libraries like `FactoryBot` switched to requiring all attributes to be passed values through a block. If we switched to something similar, the factory would look like

```crystal
class CommentFactory < Avram::Factory
def initialize
text { "not blah" }
post_id { PostFactory.create.id }
end
end
```

And when the usage I added at the very beginning is used, we could have it set up to where the block passed to `post_id` is not called resulting in no extra records made.

Thoughts?

Start by reviewing Avram::Factory and the CommentFactory/PostFactory examples described here, then trace how attribute overrides are currently evaluated. Clarify the intended block semantics with maintainers, including whether skipped defaults prevent extra records, and add coverage showing an overridden post_id does not create an additional Post.

Written by the indexing model from the issue text.