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/DataStreamIndexSettingsProvider.java
import org.elasticsearch.common.compress.CompressedXContent;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.time.DateFormatter;
import org.elasticsearch.core.CheckedFunction;
import org.elasticsearch.core.TimeValue;
import org.elasticsearch.index.IndexMode;
import org.elasticsearch.index.IndexSettingProvider;
import org.elasticsearch.index.IndexSettings;
import org.elasticsearch.index.IndexVersion;
import org.elasticsearch.index.mapper.DateFieldMapper;
import org.elasticsearch.index.mapper.KeywordFieldMapper;
import org.elasticsearch.index.mapper.Mapper;
import org.elasticsearch.index.mapper.MapperBuilderContext;
import org.elasticsearch.index.mapper.MapperService;
import org.elasticsearch.index.mapper.MappingParserContext;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.time.Instant;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Locale;
import static org.elasticsearch.cluster.metadata.IndexMetadata.INDEX_ROUTING_PATH;
/**
 * An {@link IndexSettingProvider} implementation that adds the index.time_series.start_time,
 * index.time_series.end_time and index.routing_path index settings to backing indices of
 * data streams in time series index mode.
 */
public class DataStreamIndexSettingsProvider implements IndexSettingProvider {
    static final DateFormatter FORMATTER = DateFieldMapper.DEFAULT_DATE_TIME_FORMATTER;