Add missing enum variant examples

[juhaku]

Add missing example support for enum variants. According to the docs enum variants should have support for example attribute: https://docs.rs/utoipa/latest/utoipa/derive.ToSchema.html#enum-variant-optional-configuration-options-for-schema.

This commit implements the tokenization support for example attribute at complex enum.

let value: Value = api_doc! {
    enum EnumWithExample {
        #[schema(example = "EX: Unit")]
        UnitValue,
        #[schema(example = "EX: Named")]
        NamedFields {
            #[schema(example = "EX: Named id field")]
            id: &'static str,
        },
        #[schema(example = "EX: Unnamed")]
        UnnamedFields(Foo),
    }
};

Relates #470