Custom extra validation on ParseFromJSON

[Shadow53]

Description of the feature

I'd like to be able to include extra validation beyond what's supported by OpenAPI/#[oai(validator)] as part of ParseFromJSON so that I can be sure validation always runs on received data.

I can see a few ways it could be implemented, including allowing mutation of the parsed object.

See also related issues in serde:

• Action immediately after deserialization? serde-rs/serde#626
• Add finalizer attribute hook to validate a deserialized structure serde-rs/serde#642

Code example (if possible)

#[derive(poem_openapi::Object)]
#[oai(finalize = finalize_foo)]
struct Foo {
    #[oai(validator(unique_items = true))]
    sorted: Vec<usize>,
}

// Alternatively, a signature of fn(Foo) -> ParseResult<Foo>
fn finalize_foo(parsed: &Foo) -> poem_openapi::types::ParseResult<()> {
    if !parsed.sorted.is_sorted() {
        return Err(ParseError::custom("`sorted` must be sorted"));
    }

    Ok(())
}

I'm open to providing a PR for this if the feature would be accepted.