We introduce a new type, `auth_integration`, that will be used internally
by `service_level_controller`. Its purpose is to take over the responsibility
over managing effective service levels.
The main problem of the current implementation of service level controller
is its dependency on `auth::service` whose lifetime is strictly nested
within the lifetime of service level controller. That may and already have
led to invalid memory accesses; for an example, see issue
scylladb/scylladb#24792.
Our strategy is to split service level controller into smaller parts and
ensure that we access `auth::service` only when it's valid to do so.
This commit is the first step towards that.
We don't change anything in the logic yet, just add the new type. Further
adjustments will be made in following commits.
7d0086b093