Add a dedicated auth::config struct that carries all configuration
options needed by auth modules. The config is created per-shard using
sharded_parameter to ensure updateable_value fields are shard-local.
The config is stored as a member in auth::service and passed by
const reference to factories so that each auth module can receive its
configuration when constructed. The modules themselves are not yet
converted — they still read from db::config via the query processor.
The stored config is also used in describe_roles() to read the
superuser name, eliminating the default_superuser() call that reached
into db::config via the query processor.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
