_service query should return SDL that contains schema imports (#217)

`@link` directive is used to import elements from other schema. Imported elements can be namespaced and/or renamed. Since this is required information by the gateway, it should be included in the returned `_service { sdl }` query.

Author: dariuszkuc

graphql-java-support/src/main/java/com/apollographql/federation/graphqljava/SchemaTransformer.java

@@ -139,6 +139,7 @@ public final GraphQLSchema build() throws SchemaProblem {
       sdl =
           new FederationSdlPrinter(
                   FederationSdlPrinter.Options.defaultOptions()
+                      .includeSchemaDefinition(true)
                       .includeScalarTypes(true)
                       .includeDirectiveDefinitions(
                           def -> !standardDirectives.contains(def.getName())))

graphql-java-support/src/test/resources/com/apollographql/federation/graphqljava/schemas/fed2Service.graphql

@@ -1,3 +1,7 @@
+schema @link(import : ["@key", "@shareable", {name : "@tag", as : "@apollo_tag"}, "@override"], url : "https://specs.apollo.dev/federation/v2.0"){
+  query: Query
+}
+
 directive @federation__extends on OBJECT | INTERFACE
 
 directive @federation__external on OBJECT | FIELD_DEFINITION
@@ -39,4 +43,4 @@ scalar _Any
 
 scalar federation__FieldSet
 
-scalar federation__Import
+scalar federation__Import