Add a bytes method to get the DER ASN.1 encoding of an X509 name. (#3236)

* Add a bytes method to get the DER ASN.1 encoding of an X509 name.

This is useful for creating an OpenSSL style subject_name_hash (#3011)

* add to backend interface and update multibackend

* bytes -> public_bytes
diff --git a/docs/hazmat/backends/interfaces.rst b/docs/hazmat/backends/interfaces.rst
index 0a0d145..942a359 100644
--- a/docs/hazmat/backends/interfaces.rst
+++ b/docs/hazmat/backends/interfaces.rst
@@ -585,6 +585,14 @@
         :returns: A new instance of
             :class:`~cryptography.x509.RevokedCertificate`.
 
+    .. method:: x509_name_bytes(name)
+
+        .. versionadded:: 1.6
+
+        :param name: An instance of :class:`~cryptography.x509.Name`.
+
+        :return bytes: The DER encoded bytes.
+
 .. class:: DHBackend
 
     .. versionadded:: 0.9