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
}
    private static void throwFieldNameBlank(DocumentParserContext context, String currentFieldName) {
        throw new DocumentParsingException(
            context.parser().getTokenLocation(),
            "Field name cannot contain only whitespace: [" + context.path().pathAsText(currentFieldName) + "]"
        );
    }
    private static void throwEOF(ObjectMapper mapper, DocumentParserContext context) throws IOException {
        throw new DocumentParsingException(
            context.parser().getTokenLocation(),
            "object mapping for ["
                + mapper.name()
                + "] tried to parse field ["
                + context.parser().currentName()
                + "] as object, but got EOF, has a concrete value been provided to it?"
        );
    }
    private static void copyNestedFields(DocumentParserContext context, NestedObjectMapper nested) {
        if (context.isWithinCopyTo()) {
            
            
            
            
            return;
        }
        LuceneDocument nestedDoc = context.doc();
        LuceneDocument parentDoc = nestedDoc.getParent();
        IndexVersion indexVersion = context.indexSettings().getIndexVersionCreated();
        if (nested.isIncludeInParent()) {
            addFields(indexVersion, nestedDoc, parentDoc);