Change SLM stats format

[gwbrown]

Using arrays of objects with embedded IDs is preferred for new APIs over
using entity IDs as JSON keys. This commit changes the SLM stats API to
use the preferred format.

Example of new GET _slm/stats response:

{
    "retention_runs": 0,
    "retention_failed": 0,
    "retention_timed_out": 0,
    "retention_deletion_time": "0s",
    "retention_deletion_time_millis": 0,
    "total_snapshots_taken": 1,
    "total_snapshots_failed": 0,
    "total_snapshots_deleted": 0,
    "total_snapshot_deletion_failures": 0,
    "policy_stats": [
        {
            "policy": "daily-snapshots",
            "snapshots_taken": 1,
            "snapshots_failed": 0,
            "snapshots_deleted": 0,
            "snapshot_deletion_failures": 0
        }
    ]
}

vs. old format (collapsed)

{
    "retention_runs": 0,
    "retention_failed": 0,
    "retention_timed_out": 0,
    "retention_deletion_time": "0s",
    "retention_deletion_time_millis": 0,
    "total_snapshots_taken": 1,
    "total_snapshots_failed": 0,
    "total_snapshots_deleted": 0,
    "total_snapshot_deletion_failures": 0,
    "policy_stats": {
        "daily-snapshots": {
            "snapshots_taken": 1,
            "snapshots_failed": 0,
            "snapshots_deleted": 0,
            "snapshot_deletion_failures": 0
        }
    }
}

Relates #43663

[elasticmachine]

Pinging @elastic/es-core-features

[gwbrown]

[7.5.0] Failed to apply plugin [class 'org.gradle.api.plugins.BasePlugin']
--
[7.5.0] > Could not create service of type BuildCacheController using BuildCacheServices.createBuildCacheController().

Gradle broke I guess? 🤷‍♂

@elasticmachine run elasticsearch-ci/bwc

[dakrone]

LGTM, thanks Gordon!