Allow #[schemars(with = "Type")] on struct

[Sytten]

Hi!
I am using custom serde deserializers on some of my structs to parse data. For example the string $hello.world is deserialized into:

#[derive(Clone, Debug, JsonSchema)]
pub struct Reference {
    pub left_alias: Alias,
    pub right_alias: Alias,
}

Right now we don't have a good way to tell schemars that Reference is a String without implementing JsonSchema ourselves. I would like to propose the syntax:

#[derive(Clone, Debug, JsonSchema)]
#[schemars(with = "String")]
pub struct Reference {
    ...
}

This is similar to the syntax used on fields, but allows the consumer to avoid adding it everywhere.

[Sytten]

For those interested this is the current way to do it AFAIK:

impl JsonSchema for Reference {
    fn schema_name() -> String {
        "Reference".to_owned()
    }

    fn schema_id() -> Cow<'static, str> {
        Cow::Borrowed(concat!(module_path!(), "::Reference"))
    }

    fn json_schema(gen: &mut schemars::gen::SchemaGenerator) -> Schema {
        gen.subschema_for::<String>()
    }
}