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
}
    public final ObjectMapper parent() {
        return parent;
    }
    public final MappingLookup mappingLookup() {
        return mappingLookup;
    }
    public final MetadataFieldMapper getMetadataMapper(String mapperName) {
        return mappingLookup.getMapping().getMetadataMapperByName(mapperName);
    }
    public final MappingParserContext dynamicTemplateParserContext(DateFormatter dateFormatter) {
        return mappingParserContext.createDynamicTemplateContext(dateFormatter);
    }
    public final SourceToParse sourceToParse() {
        return this.sourceToParse;
    }
    /**
     * Add the given {@code field} to the set of ignored fields.
     */
    public final void addIgnoredField(String field) {
        ignoredFields.add(field);
    }
    /**
     * Return the collection of fields that have been ignored so far.
     */
    public final Collection<String> getIgnoredFields() {
        return Collections.unmodifiableCollection(ignoredFields);
    }
    /**
     * Add the given {@code field} to the _field_names field