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
@Before
    public void setup() throws IOException {
        
        var request = new Request("POST", "/_component_template/custom_template");
        request.setJsonEntity(COMPONENT_TEMPLATE);
        assertOK(client().performRequest(request));
        
        request = new Request("POST", "/_index_template/1");
        request.setJsonEntity(TEMPLATE);
        assertOK(client().performRequest(request));
    }
    public void testTsdbDataStreams() throws Exception {
        assertTsdbDataStream();
    }
    public void testTsdbDataStreamsNanos() throws Exception {
        
        var putComposableIndexTemplateRequest = new Request("POST", "/_index_template/1");
        putComposableIndexTemplateRequest.setJsonEntity(TEMPLATE.replace("date", "date_nanos"));
        assertOK(client().performRequest(putComposableIndexTemplateRequest));
        assertTsdbDataStream();
    }
    public void testTsbdDataStreamComponentTemplateWithAllSettingsAndMappings() throws Exception {
        
        final String COMPONENT_TEMPLATE_WITH_SETTINGS_AND_MAPPINGS = """
            {
                "template": {
                    "settings":{
                        "index": {
                            "mode": "time_series",
                            "routing_path": ["metricset", "k8s.pod.uid"]