CVE-2021-20251 lib:crypto: Add md4_hash_blob() for hashing data with MD4

CVE-2021-20251 lib:crypto: Add md4_hash_blob() for hashing data with MD4

This lets us access MD4, which might not be available in hashlib, from
Python. This function is used in a following commit for hashing a
password to obtain the verifier for a SAMR password change.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14611

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>