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
import org.elasticsearch.index.IndexMode;
import org.elasticsearch.index.IndexSettings;
import org.elasticsearch.index.MapperTestUtils;
import org.elasticsearch.test.ESTestCase;
import org.junit.Before;
import java.io.IOException;
import java.time.Instant;
import java.time.temporal.ChronoUnit;
import java.util.List;
import java.util.Map;
import static org.elasticsearch.cluster.metadata.DataStreamTestHelper.createFirstBackingIndex;
import static org.elasticsearch.cluster.metadata.DataStreamTestHelper.newInstance;
import static org.elasticsearch.common.settings.Settings.builder;
import static org.elasticsearch.datastreams.DataStreamIndexSettingsProvider.FORMATTER;
import static org.hamcrest.Matchers.contains;
import static org.hamcrest.Matchers.containsInAnyOrder;
import static org.hamcrest.Matchers.equalTo;
public class DataStreamIndexSettingsProviderTests extends ESTestCase {
    private static final TimeValue DEFAULT_LOOK_AHEAD_TIME = TimeValue.timeValueHours(2); 
    DataStreamIndexSettingsProvider provider;
    @Before
    public void setup() {
        provider = new DataStreamIndexSettingsProvider(
            im -> MapperTestUtils.newMapperService(xContentRegistry(), createTempDir(), im.getSettings(), im.getIndex().getName())
        );
    }
    public void testGetAdditionalIndexSettings() throws Exception {
        Metadata metadata = Metadata.EMPTY_METADATA;
        String dataStreamName = "logs-app1";
        Instant now = Instant.now().truncatedTo(ChronoUnit.SECONDS);