fix(poem-openapi): handle additional_properties correctly in flatten

[cristicbz]

Fixes the generated documentation for a type like

#[derive(Object)]
struct SomeObject {
    fixed: u32,

    #[oai(flatten)]
    items: HashMap<String, u64>
}

Currently, such a type shows no documentation for items, because HashMap<String, ...> only contains additionalProperties in the schema. This PR adds tracking of additionalProperties in addition to properties and required when using flatten.

In principle we can only support one flattened type with additionalProperties, but it wasn't clear to me how to enforce this, so instead this PR just keeps track of the additionalProperties of the LAST flatten-ed field.