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
}
    static void additionalLookAheadTimeValidation(TimeValue lookAhead, TimeValue timeSeriesPollInterval) {
        if (lookAhead.compareTo(timeSeriesPollInterval) < 0) {
            final String message = String.format(
                Locale.ROOT,
                "failed to parse value%s for setting [%s], must be lower than setting [%s] which is [%s]",
                " [" + lookAhead.getStringRep() + "]",
                LOOK_AHEAD_TIME.getKey(),
                TIME_SERIES_POLL_INTERVAL.getKey(),
                timeSeriesPollInterval.getStringRep()
            );
            throw new IllegalArgumentException(message);
        }
    }
    @Override
    public List<Setting<?>> getSettings() {
        List<Setting<?>> pluginSettings = new ArrayList<>();
        pluginSettings.add(TIME_SERIES_POLL_INTERVAL);
        pluginSettings.add(LOOK_AHEAD_TIME);
        pluginSettings.add(DataStreamLifecycleService.DATA_STREAM_LIFECYCLE_POLL_INTERVAL_SETTING);
        pluginSettings.add(DataStreamLifecycleService.DATA_STREAM_MERGE_POLICY_TARGET_FLOOR_SEGMENT_SETTING);
        pluginSettings.add(DataStreamLifecycleService.DATA_STREAM_MERGE_POLICY_TARGET_FACTOR_SETTING);
        return pluginSettings;
    }
    @Override
    public Collection<Object> createComponents(
        Client client,
        ClusterService clusterService,
        ThreadPool threadPool,
        ResourceWatcherService resourceWatcherService,