`GraphQLObject` field attribute like serde's `serialize_with`

[DusterTheFirst]

When creating graphql objects using the GraphQLObject custom derive which have types that do not implement GraphQLType you end up being forced to use the #[juniper::object] proc macro and redefine methods to expose each and every field manually. It would be useful to have a #[graphql(transform = "fn_name")] field attribute that would allow users to transform the type into another ie. u64 (snowflake) to string.

example:

/// User id struct from library, so it is impossible to derive GraphQLType
pub struct UserId(pub u64);

#[derive(GraphQLObject)] 
struct GraphQLObj {
    normal_string: String,
    #[graphql(transform = "ToString::to_string")]
    custom_type: UserId
}

This improvement would probably be a better solution to #553.
could be a possible duplicate of #652

[DusterTheFirst]

Another solution could be to enable a way to make a type that implements Display to be usable as a GraphQLType

[tyranron]

@DusterTheFirst we need a concrete Rust type to be named for building up a GraphQL schema by introspecting the Rust code given to macros.

Unfortunately, having UserId type and ToString::to_string ident of the function, it's impossible to name the transformation result type of applying this function to UserId in the current stable Rust. If we had sort of real typeof in Rust, it would be possible. But we have no ability to name a type out of an ident.

However, if we force users to name the result type, it will work:

#[derive(GraphQLObject)] 
struct GraphQLObj {
    normal_string: String,
    #[graphql(into(String) = ToString::to_string)]
    custom_type: UserId,
}