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
assert targetDoc != null;
            final DocumentParserContext copyToContext = context.createCopyToContext(field, targetDoc);
            innerParseObject(copyToContext);
        }
    }
    
    
    
    private static Mapper getLeafMapper(final DocumentParserContext context, String fieldName) {
        Mapper mapper = context.getMapper(fieldName);
        if (mapper != null) {
            return mapper;
        }
        
        
        
        String fieldPath = context.path().pathAsText(fieldName);
        MappedFieldType fieldType = context.mappingLookup().getFieldType(fieldPath);
        if (fieldType != null) {
            
            assert fieldType.hasDocValues() == false && fieldType.isAggregatable() && fieldType.isSearchable();
            return NO_OP_FIELDMAPPER;
        }
        return null;
    }
    private static final FieldMapper NO_OP_FIELDMAPPER = new FieldMapper(
        "no-op",
        new MappedFieldType("no-op", false, false, false, TextSearchInfo.NONE, Collections.emptyMap()) {
            @Override
            public ValueFetcher valueFetcher(SearchExecutionContext context, String format) {