[Example] Embeddable by Reference and Value (#68719)

Added an attribute service to embeddable start contract which provides a higher level abstraction for embeddables that can be by reference OR by value. Added an example that uses this service.

examples/embeddable_examples/server/plugin.ts

@@ -19,10 +19,12 @@
 
 import { Plugin, CoreSetup, CoreStart } from 'kibana/server';
 import { todoSavedObject } from './todo_saved_object';
+import { bookSavedObject } from './book_saved_object';
 
 export class EmbeddableExamplesPlugin implements Plugin {
   public setup(core: CoreSetup) {
     core.savedObjects.registerType(todoSavedObject);
+    core.savedObjects.registerType(bookSavedObject);
   }
 
   public start(core: CoreStart) {}