feat(platform-server): add an API to transfer state from server (#19134)

TransferState provides a shared store that is transferred from the
server to client. To use it import BrowserTransferStateModule from the
client app module and ServerTransferStateModule from the server app
module and TransferState will be available as an Injectable object.

PR Close #19134

packages/platform-server/src/transfer_state.ts

@@ -0,0 +1,42 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {APP_ID, NgModule} from '@angular/core';
+import {DOCUMENT, TransferState, ɵescapeHtml as escapeHtml} from '@angular/platform-browser';
+
+import {BEFORE_APP_SERIALIZED} from './tokens';
+
+export function serializeTransferStateFactory(
+    doc: Document, appId: string, transferStore: TransferState) {
+  return () => {
+    const script = doc.createElement('script');
+    script.id = appId + '-state';
+    script.setAttribute('type', 'application/json');
+    script.textContent = escapeHtml(transferStore.toJson());
+    doc.body.appendChild(script);
+  };
+}
+
+/**
+ * NgModule to install on the server side while using the `TransferState` to transfer state from
+ * server to client.
+ *
+ * @experimental
+ */
+@NgModule({
+  providers: [
+    TransferState, {
+      provide: BEFORE_APP_SERIALIZED,
+      useFactory: serializeTransferStateFactory,
+      deps: [DOCUMENT, APP_ID, TransferState],
+      multi: true,
+    }
+  ]
+})
+export class ServerTransferStateModule {
+}