fix(service-worker): detect new version even if files are identical to an old one (#26006)

Previously, if an app version contained the same files as an older
version (e.g. making a change, then rolling it back), the SW would not
detect it as the latest version (and update clients).

This commit fixes it by adding a `timestamp` field in `ngsw.json`, which
makes each build unique (with sufficiently high probability).

Fixes #24338

PR Close #26006

packages/service-worker/worker/src/manifest.ts

@@ -12,6 +12,7 @@ export type ManifestHash = string;
 
 export interface Manifest {
   configVersion: number;
+  timestamp: number;
   appData?: {[key: string]: string};
   index: string;
   assetGroups?: AssetGroupConfig[];