allow cfg attributes in graphql_object

[soerenmeier]

Is your feature request related to a problem? Please describe.
I would like to enable a graphql field only if a cargo feature is set. #[cfg(feature = "myfeature")].

Describe the solution you'd like

#[graphql_object]
impl User {
    #[cfg(feature = "address")]
    fn address(&self) -> Address {
        todo!()
    }
}

Describe alternatives you've considered
As a workaround i now have two different impl blocks.

#[cfg(not(feature = "address"))]
#[graphql_object]
impl User {
    ...
}

#[cfg(feature = "address")]
#[graphql_object]
impl User {
    ...

    fn address(&self) -> Address {
        todo!()
    }
}

Thanks for considering this.

[soerenmeier]

Might be related to #600.