fix(auth): Use public refresh method for source credentials in ImpersonatedCredentials (#1884)

This PR addresses a bug in ImpersonatedCredentials that causes a issues
when the source_credential is of a type that does not implement the
private _refresh_token method (for example, a custom credential type).

Author: nbayati

google/auth/impersonated_credentials.py

@@ -286,7 +286,7 @@ def _refresh_token(self, request):
             self._source_credentials.token_state == credentials.TokenState.STALE
             or self._source_credentials.token_state == credentials.TokenState.INVALID
         ):
-            self._source_credentials._refresh_token(request)
+            self._source_credentials.refresh(request)
 
         body = {
             "delegates": self._delegates,