Add missing luci_auth property to GitCredsAuthenticator

Missed copying this in
https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6073043

Bug: b/382341041
Change-Id: I39b8ea654be603b020dd944cc8cd0404b063e496
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6082492
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>
Auto-Submit: Allen Li <ayatane@chromium.org>

gerrit_util.py

@@ -860,6 +860,10 @@ class GitCredsAuthenticator(_Authenticator):
     def __init__(self):
         self._authenticator = auth.GerritAuthenticator()
 
+    @property
+    def luci_auth(self) -> auth.Authenticator:
+        return self._authenticator
+
     def authenticate(self, conn: HttpConn):
         conn.req_headers[
             'Authorization'] = f'Bearer {self._authenticator.get_access_token()}'