perf(platform-server): use shared DomElementSchemaRegistry instance (#28150) (#28151)

Right now the `ServerRendererFactory2` creates a new instance of the
`DomElementSchemaRegistry` for each and every request, which is quite
costly (for the Tour of Heroes SSR this takes around **30%** of the
overall execution time). Since the schema is never modified, but only
used in a read-only fashion, it should be possible to re-use a single
instance instead.

Naive performance testing with 100 concurrent connections and 1000
requests in total shows an approximate **33%** improvement in Req/Sec
on the Tour of Heroes SSR example.

PR Close #28150

PR Close #28151

packages/platform-server/src/server_renderer.ts

@@ -12,11 +12,13 @@ import {DOCUMENT, EventManager, ɵNAMESPACE_URIS as NAMESPACE_URIS, ɵSharedStyl
 
 const EMPTY_ARRAY: any[] = [];
 
+const DEFAULT_SCHEMA = new DomElementSchemaRegistry();
+
 @Injectable()
 export class ServerRendererFactory2 implements RendererFactory2 {
   private rendererByCompId = new Map<string, Renderer2>();
   private defaultRenderer: Renderer2;
-  private schema = new DomElementSchemaRegistry();
+  private schema = DEFAULT_SCHEMA;
 
   constructor(
       private eventManager: EventManager, private ngZone: NgZone,