Add hash as optional attribute

[joaovbsevero]

Override pydantic's dict method to change default exclude_unset_behavior and save hash variable

[cardoso-neto]

What's the point of having an attribute called hash? I believe that this sort of thing should be managed at the user level, no at the library level. We can provide a few shortcuts to use a hash of a set of fields as primary keys at the library level, but we shouldn't interfere with a collection's attributes for that.

[joaovbsevero]

It was requested on a discord thread, I will ping you inside the thread so we can further discuss it there