add ilm explain api

[angorayc]

Summary

This is a part of https://github.com/elastic/security-team/issues/5578

Request:

GET /internal/data_quality/ilm_explain/{indexPattern}

example: GET /internal/data_quality/ilm_explain/auditbeat-*

Response:

{
    "auditbeat-7.15.1-2023.01.04-000001": {
        "index": "auditbeat-7.15.1-2023.01.04-000001",
        "managed": true,
        "policy": "auditbeat",
        "index_creation_date_millis": 1672843977429,
        "time_since_index_creation": "20.05h",
        "lifecycle_date_millis": 1672843977429,
        "age": "20.05h",
        "phase": "hot",
        "phase_time_millis": 1672843989756,
        "action": "rollover",
        "action_time_millis": 1672843998078,
        "step": "check-rollover-ready",
        "step_time_millis": 1672843998078,
        "phase_execution": {
            "policy": "auditbeat",
            "phase_definition": {
                "min_age": "0ms",
                "actions": {
                    "rollover": {
                        "max_size": "50gb",
                        "max_age": "30d"
                    }
                }
            },
            "version": 1,
            "modified_date_in_millis": 1672843969315
        }
    }
}

Checklist

Delete any items that are not applicable to this PR.

• Unit or functional tests were updated or added to match the most common scenarios

[andrew-goldstein]

Thanks @angorayc for implementing this API 🙏
✅ desk tested locally
LGTM 🚀