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/javaRestTest/java/org/elasticsearch/datastreams/TsdbDataStreamRestIT.java
String endTimeFirstBackingIndex = ObjectPath.evaluate(indices, escapedBackingIndex + ".settings.index.time_series.end_time");
        assertThat(endTimeFirstBackingIndex, notNullValue());
        var rolloverRequest = new Request("POST", "/k8s/_rollover");
        assertOK(client().performRequest(rolloverRequest));
        response = client().performRequest(getDataStreamsRequest);
        assertOK(response);
        dataStreams = entityAsMap(response);
        assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.name"), equalTo("k8s"));
        assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.generation"), equalTo(2));
        String secondBackingIndex = ObjectPath.evaluate(dataStreams, "data_streams.0.indices.1.index_name");
        assertThat(secondBackingIndex, backingIndexEqualTo("k8s", 2));
        indices = getIndex(secondBackingIndex);
        escapedBackingIndex = secondBackingIndex.replace(".", "\\.");
        assertThat(ObjectPath.evaluate(indices, escapedBackingIndex + ".data_stream"), equalTo("k8s"));
        String startTimeSecondBackingIndex = ObjectPath.evaluate(indices, escapedBackingIndex + ".settings.index.time_series.start_time");
        assertThat(startTimeSecondBackingIndex, equalTo(endTimeFirstBackingIndex));
        String endTimeSecondBackingIndex = ObjectPath.evaluate(indices, escapedBackingIndex + ".settings.index.time_series.end_time");
        assertThat(endTimeSecondBackingIndex, notNullValue());
        var indexRequest = new Request("POST", "/k8s/_doc");
        indexRequest.addParameter("refresh", "true");
        Instant time = parseInstant(startTimeFirstBackingIndex);
        indexRequest.setJsonEntity(DOC.replace("$time", formatInstantNanos(time)));
        response = client().performRequest(indexRequest);
        assertOK(response);
        assertThat(entityAsMap(response).get("_index"), equalTo(firstBackingIndex));
        indexRequest = new Request("POST", "/k8s/_doc");
        indexRequest.addParameter("refresh", "true");
        time = parseInstant(endTimeSecondBackingIndex).minusMillis(1);
        indexRequest.setJsonEntity(DOC.replace("$time", formatInstantNanos(time)));
        response = client().performRequest(indexRequest);