Update overview for fed2 (#1741)

The overview example was still in limbo between federation 1 & 2. This updates the Users schema to add `@shareable` to `User.username` and updates the corresponding supergraph schema.

docs/source/federated-types/overview.mdx

@@ -41,7 +41,7 @@ type Query {
 
 type User @key(fields: "id") {
   id: ID!
-  username: String!
+  username: String! @shareable
 }
 
 # (Subgraph schemas include
@@ -111,65 +111,78 @@ Here's the supergraph schema composed with [the subgraph schemas above](#subgrap
 
 ```graphql
 schema
-  @core(feature: "https://specs.apollo.dev/core/v0.2"),
-  @core(feature: "https://specs.apollo.dev/join/v0.1", for: EXECUTION)
+  @link(url: "https://specs.apollo.dev/link/v1.0")
+  @link(url: "https://specs.apollo.dev/join/v0.2", for: EXECUTION)
 {
   query: Query
 }
 
-directive @core(feature: String!, as: String, for: core__Purpose) repeatable on SCHEMA
-
-directive @join__field(graph: join__Graph, requires: join__FieldSet, provides: join__FieldSet) on FIELD_DEFINITION
+directive @join__field(graph: join__Graph!, requires: join__FieldSet, provides: join__FieldSet, type: String, external: Boolean, override: String, usedOverridden: Boolean) repeatable on FIELD_DEFINITION | INPUT_FIELD_DEFINITION
 
-directive @join__type(graph: join__Graph!, key: join__FieldSet) repeatable on OBJECT | INTERFACE
+directive @join__graph(name: String!, url: String!) on ENUM_VALUE
 
-directive @join__owner(graph: join__Graph!) on OBJECT | INTERFACE
+directive @join__implements(graph: join__Graph!, interface: String!) repeatable on OBJECT | INTERFACE
 
-directive @join__graph(name: String!, url: String!) on ENUM_VALUE
+directive @join__type(graph: join__Graph!, key: join__FieldSet, extension: Boolean! = false, resolvable: Boolean! = true) repeatable on OBJECT | INTERFACE | UNION | ENUM | INPUT_OBJECT | SCALAR
 
-enum core__Purpose {
-  EXECUTION
-  SECURITY
-}
+directive @link(url: String, as: String, for: link__Purpose, import: [link__Import]) repeatable on SCHEMA
 
 scalar join__FieldSet
 
 enum join__Graph {
-  USERS @join__graph(name: "users" url: "http://localhost:4001")
-  PRODUCTS @join__graph(name: "products" url: "http://localhost:4002")
-  REVIEWS @join__graph(name: "reviews" url: "http://localhost:4003")
+  PRODUCTS @join__graph(name: "products", url: "http://localhost:4003/graphql")
+  REVIEWS @join__graph(name: "reviews", url: "http://localhost:4002/graphql")
+  USERS @join__graph(name: "users", url: "http://localhost:4001/graphql")
+}
+
+scalar link__Import
+
+enum link__Purpose {
+  """
+  `SECURITY` features provide metadata necessary to securely resolve fields.
+  """
+  SECURITY
+
+  """
+  `EXECUTION` features provide metadata necessary for operation execution.
+  """
+  EXECUTION
 }
 
 type Product
-  @join__owner(graph: PRODUCTS)
   @join__type(graph: PRODUCTS, key: "upc")
   @join__type(graph: REVIEWS, key: "upc")
 {
+  upc: String!
   name: String! @join__field(graph: PRODUCTS)
   price: Int @join__field(graph: PRODUCTS)
   reviews: [Review] @join__field(graph: REVIEWS)
-  upc: String! @join__field(graph: PRODUCTS)
 }
 
-type Query {
-  me: User @join__field(graph: USERS)
+type Query
+  @join__type(graph: PRODUCTS)
+  @join__type(graph: REVIEWS)
+  @join__type(graph: USERS)
+{
   topProducts(first: Int = 5): [Product] @join__field(graph: PRODUCTS)
+  me: User @join__field(graph: USERS)
 }
 
-type Review {
-  author: User @join__field(graph: REVIEWS, provides: "username")
+type Review
+  @join__type(graph: REVIEWS)
+{
   body: String
+  author: User @join__field(graph: REVIEWS, provides: "username")
   product: Product
 }
 
 type User
-  @join__owner(graph: USERS)
-  @join__type(graph: USERS, key: "id")
   @join__type(graph: REVIEWS, key: "id")
+  @join__type(graph: USERS, key: "id")
 {
-  id: ID! @join__field(graph: USERS)
+  id: ID!
+  username: String! @join__field(graph: REVIEWS, external: true) @join__field(graph: USERS)
   reviews: [Review] @join__field(graph: REVIEWS)
-  username: String! @join__field(graph: USERS)
 }
 ```