implement a simple approach to using the link directive

this is probably the simple way to get the link directive in place for
federation 2 support - simply add a flag to the registry, and print out
a hardcoded link directive configured for the current needs of
async-graphql.

src/registry/export_sdl.rs

@@ -111,7 +111,14 @@ impl Registry {
             writeln!(sdl).ok();
         }
 
-        if !options.federation {
+        if options.federation {
+            if self.enable_apollo_link {
+                writeln!("extend schema @link(").ok();
+                writelin!("\turl: \"https://specs.apollo.dev/federation/v2.0\",").ok();
+                writeln!("\timport: [\"@key\", \"@tag\", \"@shareable\", \"@inaccessible\", \"@override\", \"@external\", \"@provides\", \"@requires\"]").ok();
+                writeln!(")").ok();
+            }
+        } else {
             writeln!(sdl, "schema {{").ok();
             writeln!(sdl, "\tquery: {}", self.query_type).ok();
             if let Some(mutation_type) = self.mutation_type.as_deref() {

src/registry/mod.rs

@@ -413,6 +413,11 @@ pub struct MetaDirective {
     pub visible: Option<MetaVisibleFn>,
 }
 
+#[derive(Debug, Clone)]
+pub struct ApolloLinkConfig {
+    pub extend_schema: bool,
+}
+
 #[derive(Default)]
 pub struct Registry {
     pub types: BTreeMap<String, MetaType>,
@@ -423,6 +428,7 @@ pub struct Registry {
     pub subscription_type: Option<String>,
     pub introspection_mode: IntrospectionMode,
     pub enable_federation: bool,
+    pub enable_apollo_link: bool,
     pub federation_subscription: bool,
     pub ignore_name_conflicts: HashSet<String>,
 }

src/schema.rs

@@ -30,6 +30,7 @@ use crate::{
     InputType, ObjectType, OutputType, QueryEnv, Request, Response, ServerError, ServerResult,
     SubscriptionType, Variables, ID,
 };
+use crate::registry::ApolloLinkConfig;
 
 /// Introspection mode
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
@@ -173,6 +174,15 @@ impl<Query, Mutation, Subscription> SchemaBuilder<Query, Mutation, Subscription>
         self
     }
 
+    /// Enables printing the apollo federation 2 `@link` directive during federation schema export;
+    /// the directive is attached to an "extend schema" element, and will have values set to ensure that
+    /// the federation schema directives and types are named properly.
+    #[must_use]
+    pub fn enable_apollo_fed2_link(mut self) -> Self {
+        self.registry.enable_apollo_link = true;
+        self
+    }
+
     /// Make the Federation SDL include subscriptions.
     ///
     /// Note: Not included by default, in order to be compatible with Apollo