Support min_items, etc. for non-array, single-type collections

[Shadow53]

Description of the feature

I'd like to be able to use min_items with HashSet<T>, but it is not currently supported. The error looks like:

Diagnostics:
the trait bound `std::collections::HashSet<MyType>: std::ops::Deref` is not satisfied
the trait `poem_openapi::Validator<T>` is implemented for `poem_openapi::validation::MinItems`
required for `poem_openapi::validation::MinItems` to implement `poem_openapi::Validator<std::collections::HashSet<MyType>>` [E0277]

I figure it makes sense for the feature request to also encompass BTreeSet and maybe tuples of a single type.

Code example (if possible)

#[derive(poem_openapi::Object)]
struct MyObject {
    #[oai(validator(min_items = 1))]
    my_set: HashSet<String>,
}

I'm willing to provide a PR if this feature would be accepted.

[attila-lin]

Yes please. PR are welcome~