…pshot of that SLM policy (elastic#100911)

The `WaitForSnapshotStep` used to check if the SLM policy has been
executed after the index has entered the delete phase, but it did not
check if the SLM policy included this index.

The result of this is that if the user used an SLM policy that did not
include this index, when the index would enter the
`WaitForSnapshotStep`, it would wait for a snapshot to be taken, a
snapshot that would not include the index, and then ILM would delete the
index.

See the exact reproduction path:
elastic#57809

**Solution** This PR, after it finds a successful SLM run, it verifies
if the snapshot taken by SLM contains this index. If not it throws an
error, otherwise it proceeds.

ILM explain will report:

```
"step_info": {
        "type": "illegal_state_exception",
        "reason": "the last successful snapshot of policy 'hourly-snapshots' does not include index '.ds-my-other-stream-2023.10.16-000001'"
      }
```

**Backwards compatibility concerns** In this PR, the
`WaitForSnapshotStep` changed from `ClusterStateWaitStep` to
`AsyncWaitStep`. We do not think this is gonna cause an issue. This was
tested manually by the following steps: - Run a master node with the old
version. - When ILM is executing `wait-for-snapshot`, we shutdown the
node - We start the node again with the new version os ES - ES was able
to pick up the step and continue with the new code.

We believe that this covers bwc concerns.

Fixes: elastic#57809