Allow specifying the Context type in #[derive(GraphQLObject)]

This commit extends the GraphQLObject derive to allow specifying a
custom context type.

integration_tests/juniper_tests/src/codegen/derive_object.rs

@@ -70,6 +70,15 @@ struct SkippedFieldObj {
     skipped: i32,
 }
 
+struct Context;
+impl juniper::Context for Context {}
+
+#[derive(GraphQLObject, Debug)]
+#[graphql(Context = "Context")]
+struct WithCustomContext {
+    a: bool,
+}
+
 graphql_object!(Query: () |&self| {
     field obj() -> Obj {
       Obj{

juniper_codegen/src/derive_object.rs

@@ -8,6 +8,7 @@ use util::*;
 struct ObjAttrs {
     name: Option<String>,
     description: Option<String>,
+    context: Option<Ident>,
     scalar: Option<Ident>,
 }
 
@@ -46,6 +47,10 @@ impl ObjAttrs {
                     res.scalar = Some(Ident::new(&scalar as &str, Span::call_site()));
                     continue;
                 }
+                if let Some(AttributeValue::String(ctx)) = keyed_item_value(&item, "Context", AttributeValidation::String) {
+                    res.context = Some(Ident::new(&ctx as &str, Span::call_site()));
+                    continue;
+                }
                 panic!(format!(
                     "Unknown struct attribute for #[derive(GraphQLObject)]: {:?}",
                     item
@@ -223,13 +228,15 @@ pub fn impl_object(ast: &syn::DeriveInput) -> TokenStream {
         .scalar
         .unwrap_or_else(|| Ident::new("__S", Span::call_site()));
 
+    let ctx = attrs.context.map(|ident| quote!( #ident )).unwrap_or(quote!( () ));
+
     let (impl_generics, _, where_clause) = generics.split_for_impl();
 
     let body = quote! {
         impl#impl_generics juniper::GraphQLType<#scalar> for #ident #ty_generics
             #where_clause
         {
-            type Context = ();
+            type Context = #ctx;
             type TypeInfo = ();
 
             fn name(_: &()) -> Option<&str> {