auth: Decouple SASL code from its parent class
auth: Decouple SASL code from its parent class

This way, we can (in the future) use this implementation of the SASL
"PLAIN" mechanism in other contexts other than `password_authenticator`.

avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla