Currently the `index.look_ahead_time` index setting controls the `index.time_series_end_time` index setting when rolling over a tsdb data stream. The `index.look_ahead_time` setting default to 2 hours, which means the by default the `index.time_series_end_time` index setting of the new backing index will be 2 hours later than the `index.time_series.start_time` index setting. The the `index.time_series.start_time` and `index.time_series_end_time` index settings  control what `@timestamp` are allowed to be indexed.

Additionally the `index.look_ahead_time` setting is used to generate the `index.time_series.start_time` and `index.time_series_end_time` index settings when creating a new data stream for the first backing index. Based on the defaults of `index.look_ahead_time` setting, the  `index.time_series.start_time` index is set to current time - 2 hours and the `index.time_series_end_time` index setting is set to current time + 2 hours.

However for indexing the initial data setting the `index.time_series.start_time` index setting to 2 hours in the past sometimes creates too small time window. For example because data gets indexed starting from midnight that day. 

The idea is to add a `index.look_back_time` setting that controls how to generate the `index.time_series.start_time` index setting when creating the data stream (for the first backing index). This allows for creating a first backing index that accepts data that is earlier then current time - $index.look_ahead_time, without affecting how `index.time_series.start_time` and `index.time_series_end_time` index settings are generated during rollover.

I think a good default for the `index.look_back_time` setting would be 24 hours. Meaning that by default the `index.time_series.start_time` index setting for the first backing index of a new data stream would be current time - 24 hours.

Relates to elastic/integrations#7345
modules/data-streams/src/main/java/org/elasticsearch/datastreams/DataStreamsPlugin.java
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
import org.elasticsearch.common.settings.ClusterSettings;
import org.elasticsearch.common.settings.IndexScopedSettings;
import org.elasticsearch.common.settings.Setting;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.settings.SettingsFilter;
import org.elasticsearch.core.IOUtils;
import org.elasticsearch.core.TimeValue;
import org.elasticsearch.datastreams.action.CreateDataStreamTransportAction;
import org.elasticsearch.datastreams.action.DataStreamsStatsTransportAction;
import org.elasticsearch.datastreams.action.DeleteDataStreamTransportAction;
import org.elasticsearch.datastreams.action.GetDataStreamsTransportAction;
import org.elasticsearch.datastreams.action.MigrateToDataStreamTransportAction;
import org.elasticsearch.datastreams.action.ModifyDataStreamsTransportAction;
import org.elasticsearch.datastreams.action.PromoteDataStreamTransportAction;
import org.elasticsearch.datastreams.lifecycle.DataStreamLifecycleErrorStore;
import org.elasticsearch.datastreams.lifecycle.DataStreamLifecycleService;
import org.elasticsearch.datastreams.lifecycle.action.DeleteDataStreamLifecycleAction;
import org.elasticsearch.datastreams.lifecycle.action.ExplainDataStreamLifecycleAction;
import org.elasticsearch.datastreams.lifecycle.action.GetDataStreamLifecycleAction;
import org.elasticsearch.datastreams.lifecycle.action.PutDataStreamLifecycleAction;
import org.elasticsearch.datastreams.lifecycle.action.TransportDeleteDataStreamLifecycleAction;
import org.elasticsearch.datastreams.lifecycle.action.TransportExplainDataStreamLifecycleAction;
import org.elasticsearch.datastreams.lifecycle.action.TransportGetDataStreamLifecycleAction;
import org.elasticsearch.datastreams.lifecycle.action.TransportPutDataStreamLifecycleAction;
import org.elasticsearch.datastreams.lifecycle.rest.RestDeleteDataStreamLifecycleAction;
import org.elasticsearch.datastreams.lifecycle.rest.RestExplainDataStreamLifecycleAction;
import org.elasticsearch.datastreams.lifecycle.rest.RestGetDataStreamLifecycleAction;
import org.elasticsearch.datastreams.lifecycle.rest.RestPutDataStreamLifecycleAction;