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
assertOK(response);
                var responseBody = entityAsMap(response);
                
                assertThat((String) responseBody.get("_index"), backingIndexEqualTo("k8s", i + 1));
            }
            var rolloverRequest = new Request("POST", "/k8s/_rollover");
            var rolloverResponse = client().performRequest(rolloverRequest);
            assertOK(rolloverResponse);
            var rolloverResponseBody = entityAsMap(rolloverResponse);
            assertThat(rolloverResponseBody.get("rolled_over"), is(true));
        }
        var getDataStreamsRequest = new Request("GET", "/_data_stream");
        var getDataStreamResponse = client().performRequest(getDataStreamsRequest);
        assertOK(getDataStreamResponse);
        var dataStreams = entityAsMap(getDataStreamResponse);
        assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.name"), equalTo("k8s"));
        assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.generation"), equalTo(5));
        for (int i = 0; i < 5; i++) {
            String backingIndex = ObjectPath.evaluate(dataStreams, "data_streams.0.indices." + i + ".index_name");
            assertThat(backingIndex, backingIndexEqualTo("k8s", i + 1));
            var indices = getIndex(backingIndex);
            var escapedBackingIndex = backingIndex.replace(".", "\\.");
            assertThat(ObjectPath.evaluate(indices, escapedBackingIndex + ".data_stream"), equalTo("k8s"));
            assertThat(ObjectPath.evaluate(indices, escapedBackingIndex + ".settings.index.mode"), nullValue());
            assertThat(ObjectPath.evaluate(indices, escapedBackingIndex + ".settings.index.time_series.start_time"), nullValue());
            assertThat(ObjectPath.evaluate(indices, escapedBackingIndex + ".settings.index.time_series.end_time"), nullValue());
        }
        
        putComposableIndexTemplateRequest = new Request("POST", "/_index_template/1");
        putComposableIndexTemplateRequest.setJsonEntity(TEMPLATE);