Add a snapshot test module to delay shard aggregations

[jimczi]

This change adds an aggregation that can be used to delay the
query phase execution on shards with a configurable time:

{
    "aggs": {
        "delay": {
            "shard_delay": {
                "value": "30s"
            },
            "aggs": {
                "host": {
                    "terms": {
                        "field": "hostname"
                    }
                }
            }
        }
    }
}

This test module is built on top of #61954 so the aggregation will be available only within
snapshots since this module is not meant to be used in production.

Closes #54159

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Build)

[elasticmachine]

Pinging @elastic/es-search (:Search/Search)

[jpountz]

I like the simplicity of the approach. The reliance on wall clock means that you can't do much more automated testing besides making sure that everything keeps working with slow response times, but this still feels useful to me, and even more useful for manual testing.