Make pointers and/or `OmittableNullable` nullable in the generated OpenAPI spec

[gregoryjjb]

How can I set fields to be nullable in the generated spec? I can't find any way to set nullability on a Schema.

Ideally I'd like to make pointers always nullable (to match my Go code as closely as possible), and/or have custom types like the OmittableNullable example generate as nullable.

Thanks in advance ❤️