feat(2639): implement entity resolver

examples/apollo_federation_subgraph_post.graphql

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+schema
+  @server(port: 8001)
+  @upstream(baseURL: "http://jsonplaceholder.typicode.com", httpCache: 42, batch: {delay: 100}) {
+  query: Query
+}
+
+type Query {
+  posts: [Post] @http(path: "/posts")
+}
+
+type User @http(path: "/users", query: [{key: "id", value: "{{.value.id}}"}], batchKey: ["id"]) {
+  id: Int!
+}
+
+type Post @http(path: "/posts", query: [{key: "id", value: "{{.value.id}}"}], batchKey: ["id"]) {
+  id: Int!
+  userId: Int!
+  title: String!
+  body: String!
+  user: User @expr(body: {id: "{{.value.userId}}"})
+}

examples/apollo_federation_subgraph_user.graphql

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+schema
+  @server(port: 8002)
+  @upstream(baseURL: "http://jsonplaceholder.typicode.com", httpCache: 42, batch: {delay: 100}) {
+  query: Query
+}
+
+type Query {
+  user(id: Int!): User @http(path: "/users/{{.args.id}}")
+}
+
+type User @http(path: "/users", query: [{key: "id", value: "{{.value.id}}"}], batchKey: ["id"]) {
+  id: Int!
+  name: String
+  username: String
+  email: String
+  phone: String
+  website: String
+}

examples/federation/README.md

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+# Apollo Federation example
+
+1. Start tailcall subgraph examples:
+
+- `cargo run -- start examples/apollo_federation_subgraph_post.graphql`
+- `cargo run -- start examples/apollo_federation_subgraph_user.graphql`
+
+2. Run Apollo router by one of the following methods:
+
+- run `@apollo/gateway` with `npm start` (with `npm install` for the first time) from "examples/federation" folder
+- start apollo router with `rover.sh` script (install [apollo rover](https://www.apollographql.com/docs/rover) first)
+
+3. Navigate to `http://localhost:4000` and execute supergraph queries, see [examples](#query-examples)
+
+# Query examples
+
+```graphql
+{
+  posts {
+    id
+    title
+    user {
+      id
+      name
+    }
+  }
+}
+```

examples/federation/gateway.js

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+import {ApolloServer} from "@apollo/server"
+import {startStandaloneServer} from "@apollo/server/standalone"
+import {ApolloGateway, IntrospectAndCompose} from "@apollo/gateway"
+
+const gateway = new ApolloGateway({
+  supergraphSdl: new IntrospectAndCompose({
+    subgraphs: [
+      {name: "post", url: "http://localhost:8001/graphql"},
+      {name: "user", url: "http://localhost:8002/graphql"},
+    ],
+  }),
+})
+
+const server = new ApolloServer({
+  gateway,
+  introspection: true,
+})
+
+const {url} = await startStandaloneServer(server)
+console.log(`🚀  Server ready at ${url}`)