Added a HTTP backend for the authentication mechanism `scram`.

Note: This is not an implementation of the RFC 7804: Salted Challenge Response HTTP Authentication Mechanism.

This backend is an implementation of scram that uses an external web resource as a source of SCRAM authentication data, including stored key of the client, server key, and the salt. It support other authentication and authorization extension fields like HTTP auth backend, namely: `is_superuser`, `client_attrs`, `expire_at` and `acl`.