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: https://github.com/poem-web/poem/issues/701