add OCSP basic response extension parsing (#4479)

* add OCSP basic response extension parsing

Just nonce for now. This does not support SINGLERESP extension parsing.

* also raises on extensions for non-successful

* empty commit
diff --git a/docs/x509/ocsp.rst b/docs/x509/ocsp.rst
index b203022..bf06413 100644
--- a/docs/x509/ocsp.rst
+++ b/docs/x509/ocsp.rst
@@ -395,6 +395,11 @@
         :raises ValueError: If ``response_status`` is not
             :class:`~cryptography.x509.ocsp.OCSPResponseStatus.SUCCESSFUL`.
 
+    .. attribute:: extensions
+
+        :type: :class:`~cryptography.x509.Extensions`
+
+        The extensions encoded in the response.
 
 .. class:: OCSPResponseStatus