feat: sign() on AuthClient (#109)

* refactor(dart): apply DRY pattern to HTTP clients for consistent client/projectId handling

Add `_run` helper method to MessagingHttpClient, SecurityRulesHttpClient, and AppCheckHttpClient that accepts both client and projectId as callback parameters. This eliminates redundant `await app.client` calls and centralizes client/projectId retrieval logic.

* refactor: update exception constructors to use FirebaseServiceType for consistency

* test: add AppRegistry tests for singleton behavior and environment options

* feat: implement sign() extension on AuthClient with IAM Credentials API, local RSA signing, and service account impersonation

* refactor: delegate credential implementation to googleapis_auth_utils

* refactor: use GoogleCredential to parse service account files

* refactor(auth): use Expando to associate credentials with AuthClient

* feat(auth): add getAccessToken() method to GoogleCredential for OAuth2 token retrieval

* chore: update googleapis dependency to version 15.0.0 and add coverage.lcov to .gitignore

* refactor: sign method to use CryptoSigner with custom endpoint.

* feat: add universeDomain support for GoogleCredential and improve IAM endpoint handling

Author: demolaf

.gitignore

@@ -1,6 +1,7 @@
 firebase-debug.log
 ui-debug.log
 firestore-debug.log
+coverage.lcov
 
 node_modules
 packages/dart_firebase_admin/test/client/package-lock.json