Control storing array source with index setting (#112397)

Introduce an index setting that forces storing the source of leaf field
and object arrays in synthetic source mode. Nested objects are excluded
as they already preserve ordering in synthetic source.

Next step is to introduce override params at the mapper level that will
allow disabling the source, or storing the source for arrays (if not
enabled at index level), or storing the source for both arrays and
singletons. This will happen in follow-up changes, so that we can
benchmark the impact of this change in parallel.

Related to #112012

Author: kkrik-es

docs/changelog/112397.yaml

@@ -0,0 +1,5 @@
+pr: 112397
+summary: Control storing array source with index setting
+area: Mapping
+type: enhancement
+issues: []

modules/data-streams/src/javaRestTest/java/org/elasticsearch/datastreams/logsdb/qa/StandardVersusLogsIndexModeRandomDataChallengeRestIT.java

@@ -8,9 +8,11 @@
 
 package org.elasticsearch.datastreams.logsdb.qa;
 
+import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.time.DateFormatter;
 import org.elasticsearch.common.time.FormatNames;
 import org.elasticsearch.core.CheckedConsumer;
+import org.elasticsearch.index.mapper.Mapper;
 import org.elasticsearch.index.mapper.ObjectMapper;
 import org.elasticsearch.logsdb.datageneration.DataGenerator;
 import org.elasticsearch.logsdb.datageneration.DataGeneratorSpecification;
@@ -36,12 +38,14 @@
  */
 public class StandardVersusLogsIndexModeRandomDataChallengeRestIT extends StandardVersusLogsIndexModeChallengeRestIT {
     private final ObjectMapper.Subobjects subobjects;
+    private final boolean keepArraySource;
 
     private final DataGenerator dataGenerator;
 
     public StandardVersusLogsIndexModeRandomDataChallengeRestIT() {
         super();
         this.subobjects = randomFrom(ObjectMapper.Subobjects.values());
+        this.keepArraySource = randomBoolean();
 
         var specificationBuilder = DataGeneratorSpecification.builder().withFullyDynamicMapping(randomBoolean());
         if (subobjects != ObjectMapper.Subobjects.ENABLED) {
@@ -120,6 +124,13 @@ public void contenderMappings(XContentBuilder builder) throws IOException {
         }
     }
 
+    @Override
+    public void contenderSettings(Settings.Builder builder) {
+        if (keepArraySource) {
+            builder.put(Mapper.SYNTHETIC_SOURCE_KEEP_INDEX_SETTING.getKey(), "arrays");
+        }
+    }
+
     @Override
     protected XContentBuilder generateDocument(final Instant timestamp) throws IOException {
         var document = XContentFactory.jsonBuilder();