This add support to the `GET _data_stream` API for displaying the value
of the `index.lifecycle.prefer_ilm` setting both at the backing index
level and at the top level (top level meaning, similarly to the existing
`ilm_policy` field, the value in the index template that's backing the
data stream), an `ilm_policy` field for each backing index displaying
the actual ILM policy configured for the index itself, a `managed_by`
field for each backing index indicating who manages this index (the
possible values are: `Index Lifecycle Management`, `Data stream
lifecycle`, and `Unmanaged`).
This also adds a top level field to indicate which system would manage
the next generation index for this data stream based on the current
configuration. This field is called `next_generation_managed_by` and the
same values as the indices level `managed_by` field has are available.
An example output for a data stream that has 2 backing indices managed
by ILM and the write index by DSL:
```
{
"data_streams": [{
"name": "datastream-psnyudmbitp",
"timestamp_field": {
"name": "@timestamp"
},
"indices": [{
"index_name": ".ds-datastream-psnyudmbitp-2023.09.27-000001",
"index_uuid": "kyw0WEXvS8-ahchYS10NRQ",
"prefer_ilm": true,
"ilm_policy": "policy-uVBEI",
"managed_by": "Index Lifecycle Management"
}, {
"index_name": ".ds-datastream-psnyudmbitp-2023.09.27-000002",
"index_uuid": "pDLdc4DERwO54GRzDr4krw",
"prefer_ilm": true,
"ilm_policy": "policy-uVBEI",
"managed_by": "Index Lifecycle Management"
}, {
"index_name": ".ds-datastream-psnyudmbitp-2023.09.27-000003",
"index_uuid": "gYZirLKcS3mlc1c3oHRpYw",
"prefer_ilm": false,
"ilm_policy": "policy-uVBEI",
"managed_by": "Data stream lifecycle"
}],
"generation": 3,
"status": "YELLOW",
"template": "indextemplate-obcvkbjqand",
"lifecycle": {
"enabled": true,
"data_retention": "90d"
},
"ilm_policy": "policy-uVBEI",
"next_generation_managed_by": "Data stream lifecycle",
"prefer_ilm": false,
"hidden": false,
"system": false,
"allow_custom_routing": false,
"replicated": false
}]
}
```
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