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
return components;
    }
    @Override
    public List<ActionHandler<? extends ActionRequest, ? extends ActionResponse>> getActions() {
        List<ActionHandler<? extends ActionRequest, ? extends ActionResponse>> actions = new ArrayList<>();
        actions.add(new ActionHandler<>(CreateDataStreamAction.INSTANCE, CreateDataStreamTransportAction.class));
        actions.add(new ActionHandler<>(DeleteDataStreamAction.INSTANCE, DeleteDataStreamTransportAction.class));
        actions.add(new ActionHandler<>(GetDataStreamAction.INSTANCE, GetDataStreamsTransportAction.class));
        actions.add(new ActionHandler<>(DataStreamsStatsAction.INSTANCE, DataStreamsStatsTransportAction.class));
        actions.add(new ActionHandler<>(MigrateToDataStreamAction.INSTANCE, MigrateToDataStreamTransportAction.class));
        actions.add(new ActionHandler<>(PromoteDataStreamAction.INSTANCE, PromoteDataStreamTransportAction.class));
        actions.add(new ActionHandler<>(ModifyDataStreamsAction.INSTANCE, ModifyDataStreamsTransportAction.class));
        actions.add(new ActionHandler<>(PutDataStreamLifecycleAction.INSTANCE, TransportPutDataStreamLifecycleAction.class));
        actions.add(new ActionHandler<>(GetDataStreamLifecycleAction.INSTANCE, TransportGetDataStreamLifecycleAction.class));
        actions.add(new ActionHandler<>(DeleteDataStreamLifecycleAction.INSTANCE, TransportDeleteDataStreamLifecycleAction.class));
        actions.add(new ActionHandler<>(ExplainDataStreamLifecycleAction.INSTANCE, TransportExplainDataStreamLifecycleAction.class));
        return actions;
    }
    @Override
    public List<RestHandler> getRestHandlers(
        Settings settings,
        RestController restController,
        ClusterSettings clusterSettings,
        IndexScopedSettings indexScopedSettings,
        SettingsFilter settingsFilter,
        IndexNameExpressionResolver indexNameExpressionResolver,
        Supplier<DiscoveryNodes> nodesInCluster
    ) {
        indexScopedSettings.addSettingsUpdateConsumer(LOOK_AHEAD_TIME, value -> {
            TimeValue timeSeriesPollInterval = updateTimeSeriesRangeService.get().pollInterval;