[backup] Extend the snapshot schema dump with hashed passwords

[karol-kokoszka]

Update the schema dump with information about the users passwords.
This can be achieved by executing "DESCRIBE SCHEMA WITH INTERNALS AND PASSWORDS".

See the documentation https://opensource.docs.scylladb.com/master/cql/describe-schema.html
It describes and explains how the role with hashed password is saved to CQL statement.

UPDATE:
If the query fails due to the missing permissions, it should fail-back to regular "DESCRIBE SCHEMA WITH INTERNALS". It basically means, that hashed passwords won't be saved then.