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/test/java/org/elasticsearch/datastreams/DataStreamIndexSettingsProviderTests.java
Instant currentEnd = sixHoursAgo.plusMillis(lookAheadTime.getMillis());
        Metadata metadata = DataStreamTestHelper.getClusterStateWithDataStream(
            dataStreamName,
            List.of(new Tuple<>(sixHoursAgo, currentEnd))
        ).getMetadata();
        Instant now = sixHoursAgo.plus(6, ChronoUnit.HOURS);
        Settings settings = Settings.EMPTY;
        var result = provider.getAdditionalIndexSettings(
            DataStream.getDefaultBackingIndexName(dataStreamName, 1),
            dataStreamName,
            true,
            metadata,
            now,
            settings,
            List.of(new CompressedXContent("{}"))
        );
        assertThat(result.size(), equalTo(2));
        assertThat(result.get(IndexSettings.TIME_SERIES_START_TIME.getKey()), equalTo(FORMATTER.format(currentEnd)));
        assertThat(
            result.get(IndexSettings.TIME_SERIES_END_TIME.getKey()),
            equalTo(FORMATTER.format(now.plusMillis(lookAheadTime.getMillis())))
        );
    }
    public void testGetAdditionalIndexSettingsDataStreamAlreadyCreatedTimeSettingsMissing() {
        String dataStreamName = "logs-app1";
        Instant twoHoursAgo = Instant.now().minus(4, ChronoUnit.HOURS).truncatedTo(ChronoUnit.MILLIS);
        Metadata.Builder mb = Metadata.builder(
            DataStreamTestHelper.getClusterStateWithDataStreams(
                List.of(Tuple.tuple(dataStreamName, 1)),
                List.of(),