Reindex: fix reindex-from-remote from <2.0 (#22931)

In 5.2 we stopped sending the source parameter if the user didn't
specify it. This was a mistake as versions before 2.0 look like
they don't always include the `_source`. This is because reindex
requests some metadata fields. Anyway, now we say `"_source": true`
if there isn't a `_source` configured in the reindex request.

Closes #22893

modules/reindex/src/main/java/org/elasticsearch/index/reindex/remote/RemoteRequestBuilders.java

@@ -124,6 +124,8 @@ final class RemoteRequestBuilders {
 
             if (searchRequest.source().fetchSource() != null) {
                 entity.field("_source", searchRequest.source().fetchSource());
+            } else {
+                entity.field("_source", true);
             }
 
             entity.endObject();

modules/reindex/src/test/java/org/elasticsearch/index/reindex/remote/RemoteRequestBuildersTests.java

@@ -160,7 +160,7 @@ public class RemoteRequestBuildersTests extends ESTestCase {
         String query = "{\"match_all\":{}}";
         HttpEntity entity = initialSearchEntity(searchRequest, new BytesArray(query));
         assertEquals(ContentType.APPLICATION_JSON.toString(), entity.getContentType().getValue());
-        assertEquals("{\"query\":" + query + "}",
+        assertEquals("{\"query\":" + query + ",\"_source\":true}",
                 Streams.copyToString(new InputStreamReader(entity.getContent(), StandardCharsets.UTF_8)));
 
         // Source filtering is included if set up