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
var response = client().performRequest(indexRequest);
            assertOK(response);
        }
        {
            var putComposableIndexTemplateRequest = new Request("POST", "/_index_template/1");
            putComposableIndexTemplateRequest.setJsonEntity(NON_TSDB_TEMPLATE);
            client().performRequest(putComposableIndexTemplateRequest);
        }
        {
            {
                
                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.indices"), hasSize(1));
                assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.time_series"), notNullValue());
            }
            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);