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/DocumentParserContext.java
mappingLookup.checkFieldLimit(indexSettings().getMappingTotalFieldsLimit(), newFieldsSeen.size());
        }
        if (mapper instanceof ObjectMapper objectMapper) {
            dynamicObjectMappers.put(objectMapper.name(), objectMapper);
            
            
            for (Mapper submapper : objectMapper.mappers.values()) {
                
                
                
                addDynamicMapper(submapper);
            }
        }
        
        
        
        
        
        
        dynamicMappers.add(mapper);
    }
    /**
     * Get dynamic mappers created as a result of parsing an incoming document. Responsible for exposing all the newly created
     * fields that need to be merged into the existing mappings. Used to create the required mapping update at the end of document parsing.
     * Consists of a flat set of {@link Mapper}s that will need to be added to their respective parent {@link ObjectMapper}s in order
     * to become part of the resulting dynamic mapping update.
     */
    public final List<Mapper> getDynamicMappers() {
        return dynamicMappers;
    }