Implemented nullable fields for openapi spec generation

[skytz]

I've added the ability to set nullable fields on a struct with poem_api::Object derive.

Following the spec here: https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.0.md#fixed-fields-20

Either for each individual field, with nullable attibute, or at the struct level with nullable_all.

It also solves this opened issue: #701

[kidqueb]

Gave this a shot and it worked great, thank you. Hopefully get this gets merged in soon.

Edit:
Revisiting, I get the limitation now. Trying to get it to set optional/nullable fields to Some(None) for null, Some(Some(T)) for the value and None when its completely omitted..

Edit edit:
Found MaybeUndefined and we're back to working great

[sunli829]

thanks