feat(platform-server): add API to render Module and ModuleFactory to string (#14381)

- PlatformState provides an interface to serialize the current Platform State as a string or Document.

- renderModule and renderModuleFactory are convenience methods to wait for Angular Application to stabilize and then render the state to a string.

- refactor code to remove defaultDoc from DomAdapter and inject DOCUMENT where it's needed.

tools/public_api_guard/platform-server/index.d.ts

@@ -1,9 +1,25 @@
+/** @experimental */
+export declare const INITIAL_CONFIG: InjectionToken<PlatformConfig>;
+
 /** @experimental */
 export declare const platformDynamicServer: (extraProviders?: Provider[]) => PlatformRef;
 
 /** @experimental */
 export declare const platformServer: (extraProviders?: Provider[]) => PlatformRef;
 
+/** @experimental */
+export declare class PlatformState {
+    constructor(_doc: any);
+    getDocument(): any;
+    renderToString(): string;
+}
+
+/** @experimental */
+export declare function renderModule<T>(module: Type<T>, options: PlatformOptions): Promise<string>;
+
+/** @experimental */
+export declare function renderModuleFactory<T>(moduleFactory: NgModuleFactory<T>, options: PlatformOptions): Promise<string>;
+
 /** @experimental */
 export declare class ServerModule {
 }