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
twoHoursAgo.toEpochMilli(),
                builder().build(),
                1
            ).getMetadata()
        );
        DataStream ds = mb.dataStream(dataStreamName);
        mb.put(
            new DataStream(
                ds.getName(),
                ds.getIndices(),
                ds.getGeneration(),
                ds.getMetadata(),
                ds.isHidden(),
                ds.isReplicated(),
                ds.isSystem(),
                ds.isAllowCustomRouting(),
                IndexMode.TIME_SERIES,
                ds.getLifecycle()
            )
        );
        Metadata metadata = mb.build();
        Instant now = twoHoursAgo.plus(2, ChronoUnit.HOURS);
        Settings settings = Settings.EMPTY;
        Exception e = expectThrows(
            IllegalStateException.class,
            () -> provider.getAdditionalIndexSettings(
                DataStream.getDefaultBackingIndexName(dataStreamName, 1),
                dataStreamName,
                true,
                metadata,