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
ScriptService scriptService,
        NamedXContentRegistry xContentRegistry,
        Environment environment,
        NodeEnvironment nodeEnvironment,
        NamedWriteableRegistry namedWriteableRegistry,
        IndexNameExpressionResolver indexNameExpressionResolver,
        Supplier<RepositoriesService> repositoriesServiceSupplier,
        Tracer tracer,
        AllocationService allocationService,
        IndicesService indicesService
    ) {
        Collection<Object> components = new ArrayList<>();
        var updateTimeSeriesRangeService = new UpdateTimeSeriesRangeService(environment.settings(), threadPool, clusterService);
        this.updateTimeSeriesRangeService.set(updateTimeSeriesRangeService);
        components.add(this.updateTimeSeriesRangeService.get());
        errorStoreInitialisationService.set(new DataStreamLifecycleErrorStore());
        dataLifecycleInitialisationService.set(
            new DataStreamLifecycleService(
                settings,
                new OriginSettingClient(client, DATA_STREAM_LIFECYCLE_ORIGIN),
                clusterService,
                getClock(),
                threadPool,
                threadPool::absoluteTimeInMillis,
                errorStoreInitialisationService.get()
            )
        );
        dataLifecycleInitialisationService.get().init();
        components.add(errorStoreInitialisationService.get());
        components.add(dataLifecycleInitialisationService.get());