Refactor: Replace AuthBlockingTokenVerifier with Admin SDK verifier (blocked by firebase/firebase-admin-dart#299)

### Background

Currently, `firebase_functions` maintains a standalone 280-line JWT verifier ([`AuthBlockingTokenVerifier`](https://github.com/firebase/firebase-functions-dart/blob/main/lib/src/identity/token_verifier.dart)) for Identity Platform Auth Blocking events.

This class duplicates JWKS fetching against `https://www.googleapis.com/oauth2/v3/certs`, HTTP `Cache-Control` header parsing, public key caching, and RS256 signature verification—mechanics that already exist in `firebase_admin_sdk` (which `firebase_functions` depends on).

### Problem

Maintaining a custom verifier has led to heuristic drift between Functions and Admin SDK:
* Different `Cache-Control` parsing implementations (regex vs. string splitting vs. ignored).
* Different default cache TTLs (1 hour in Functions vs. 6 hours in Admin JWKS).
* Clock-skew allowances (5 minutes on `iat`) present in Functions but absent in Admin SDK.

### Proposed Action Items

* [ ] Blocked by upstream Admin SDK refactoring (see https://github.com/firebase/firebase-admin-dart/issues/299).
* [ ] Once `firebase_admin_sdk` exposes a unified, configurable JWKS verifier with clock-skew and polymorphic audience support, deprecate and remove `AuthBlockingTokenVerifier`.
* [ ] Re-implement Auth Blocking token verification by delegating directly to the Admin SDK verifier, eliminating ~280 lines of duplicated cryptography and networking code.

Start by reading lib/src/identity/token_verifier.dart and the blocked upstream issue firebase/firebase-admin-dart#299. Confirm that the Admin SDK exposes the required configurable JWKS verifier with clock-skew and polymorphic audience support. Done means AuthBlockingTokenVerifier is deprecated and removed, with Auth Blocking verification delegated to the Admin SDK.

Written by the indexing model from the issue text.