feat: initializeApp idempotency (#2947)

Update `initializeApp` to return an existing app if one exists with the same name and the same `AppOptions`. However, due to their inabilty to be compared, `initializeApp` will throw if an existing app has a configured `Credential` or `httpAgent`.

Author: DellaBitta

etc/firebase-admin.app.api.md

@@ -83,10 +83,10 @@ export interface FirebaseError {
     toJSON(): object;
 }
 
-// @public (undocumented)
+// @public
 export function getApp(appName?: string): App;
 
-// @public (undocumented)
+// @public
 export function getApps(): App[];
 
 // @public
@@ -97,7 +97,7 @@ export interface GoogleOAuthAccessToken {
     expires_in: number;
 }
 
-// @public (undocumented)
+// @public
 export function initializeApp(options?: AppOptions, appName?: string): App;
 
 // @public

package-lock.json

@@ -209,6 +209,7 @@
     "@firebase/database-types": "^1.0.6",
     "@types/node": "^22.8.7",
     "farmhash-modern": "^1.1.0",
+    "fast-deep-equal": "^3.1.1",
     "google-auth-library": "^9.14.2",
     "jsonwebtoken": "^9.0.0",
     "jwks-rsa": "^3.1.0",