Refactor object proc macro attributes to support parameter attributes

[theduke]

Function parameter attributes are getting stabilized with 1.39. (see rust-lang/rust#60406 (comment)).

This means we can refactor the argument customization of the object proc macro.

This is currently awkward due to the lack of above feature ( see https://graphql-rust.github.io/juniper/current/types/objects/complex_fields.html#customizing-arguments ).

• Supported arguments: #[graphql(rename = "...", default = expr, description = "...")]
• Old style should remain supported for now but with a deprecation warning (simple println in the macro)
• can be implemented right now with a feature flag, which can be removed once 1.39 is released

[davidpdrsn]

I have started hacking in this. Progress is here https://github.com/davidpdrsn/juniper/tree/parameter-attributes

[theduke]

@davidpdrsn awesome, please ask if you have any questions.

Also feel free to open a draft PR to give it visibility.

[tyranron]

In #682 landed the implementation for GraphQL interfaces.

[tyranron]

In #971 landed the implementation for GraphQL objects and subscriptions.