Feature: Add Error with Extension in Validator

[cptrodgers]

Hi guys, this is my first contribution, please correct me.
My PR to help error data of input validator can contain more information by ErrorExtentionValues.

There are some use cases that I can imagine:

• Use code number or code string for show exactly error (Bad Request, Internal Server Error) on Client Side.
• Support localization (i18n) by adding key and its data on extensions.
• ...

Example implementation:

    use async_graphql::*;
    pub struct IntGreaterThanZero;

    impl InputValueValidator for IntGreaterThanZero {
        fn is_valid_with_extensions(&self, value: &Value) -> Result<(), Error> {
            if let Value::Number(n) = value {
                if let Some(n) = n.as_i64() {
                    if n <= 0 {
                        let e = Error::new("Value must be greater than 0")
                            .extend_with(|_, e| e.set("code", 400));
                        return Err(e);
                    }
                }
            }
            Ok(())
        }
    }

Expected Output:

{
  "data": null,
  "errors": [
    {
      "message": "... Value must be greater than 0",
      "locations": [],
      "extensions": { "code": 400 }
    }
  ]
}

Compatible note:

• Compatible with all logic code of fn is_valid.
• Break MapErr logic on src/validators/mod.rs. Closure from fn(String) -> String to fn(Error) -> Error. Compatible.

[cptrodgers]

cc @sunli829 . Feel free to correct me. Thanks.

[sunli829]

Thank you very much, it looks great!
If you are free, please help update CHANGELOG. 🙂

[cptrodgers]

Thank you very much, it looks great!
If you are free, please help update CHANGELOG. 🙂

Updated. Thanks @sunli829 .