chore: add initial common model interfaces

Author: EdricChan03

src/app/data/models/common/id.ts

@@ -0,0 +1,4 @@
+export interface HasId {
+  /** Unique ID of this document. */
+  id: string;
+}

src/app/data/models/dto/metadata.ts

@@ -0,0 +1,10 @@
+import { Timestamp } from '@firebase/firestore';
+
+export { HasId } from '../common/id';
+
+export interface WithDbTimestampMetadata {
+  /** Creation date of this document as a Firestore {@link Timestamp}. */
+  createdAt: Timestamp;
+  /** Last modification date of this document as a Firestore {@link Timestamp}. */
+  lastModified: Timestamp;
+}