Currently, when an object is mapped with `subobjects: false`, trying to ingest a document that has nested objects leads to a mapping exception.

In a couple of occasions, we've been talking about that it would be convenient if Elasticsearch could automatically flatten the fields instead of the client having to know about the `subobjects` setting. This would make `subobjects: false` more of a drop-in replacement that allows for a much easier adoption, without changes in the client.

Note that the `_source` should ideally stay untouched, just the stored fields should be flattened automatically.

Some related issues
* https://github.com/elastic/apm/issues/347
* https://github.com/elastic/elasticsearch/issues/88777
* https://github.com/elastic/package-spec/issues/349
server/src/main/java/org/elasticsearch/index/mapper/DocumentParser.java
metadataMapper.postParse(context);
            }
        } catch (Exception e) {
            throw wrapInDocumentParsingException(context, e);
        }
    }
    private static void executeIndexTimeScripts(DocumentParserContext context) {
        List<FieldMapper> indexTimeScriptMappers = context.mappingLookup().indexTimeScriptMappers();
        if (indexTimeScriptMappers.isEmpty()) {
            return;
        }
        SearchLookup searchLookup = new SearchLookup(
            context.mappingLookup().indexTimeLookup()::get,
            (ft, lookup, fto) -> ft.fielddataBuilder(
                new FieldDataContext(context.indexSettings().getIndex().getName(), lookup, context.mappingLookup()::sourcePaths, fto)
            ).build(new IndexFieldDataCache.None(), new NoneCircuitBreakerService()),
            (ctx, doc) -> Source.fromBytes(context.sourceToParse().source())
        );
        
        
        
        Map<String, Consumer<LeafReaderContext>> fieldScripts = new HashMap<>();
        indexTimeScriptMappers.forEach(mapper -> fieldScripts.put(mapper.name(), new Consumer<>() {
            boolean executed = false;
            @Override
            public void accept(LeafReaderContext leafReaderContext) {
                if (executed == false) {
                    mapper.executeScript(searchLookup, leafReaderContext, 0, context);
                    executed = true;
                }