[ILM] Keep track of ILM steps and actions executed

[lucabelluccini]

Describe the feature:

The _ilm/explain API is really useful when troubleshooting, but unfortunately has no way to keep track "past" action execution events.

It would be nice to have one of the following:

1. The timestamp within the output of the _ilm/explain API for each step and action

It is probably not feasible because of the fact this gets stored in the cluster state and if the index is deleted, we do not have any history.
It would mean N additional epoch fields, one per step/action. This could be simplified if we restrict this "history" to steps only.

2. An hidden index (e.g. .ilm-history) to keep track of the executed ILM steps and actions

3. An "audit" log for ILM

Somehow similar to #40552, but it's output is quite technical/verbose and requires debug/trace logging.

---

On the same subject, it would be interesting to put the chosen node ID when shrinking an index (in the shrink action) in the ILM explain API.
This could be a small extra information which would be present only on indices being shrunk.

FYI @leehinman

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/ILM+SLM)