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
/**
     * Find a dynamic mapping template for the given field and its matching type
     *
     * @param fieldName the name of the field
     * @param matchType the expecting matchType of the field
     * @return the matching template; otherwise returns null
     * @throws DocumentParsingException if the given field has a dynamic template name specified, but no template matches that name.
     */
    public final DynamicTemplate findDynamicTemplate(String fieldName, DynamicTemplate.XContentFieldType matchType) {
        final String pathAsString = path().pathAsText(fieldName);
        final String matchTemplateName = sourceToParse().dynamicTemplates().get(pathAsString);
        for (DynamicTemplate template : root().dynamicTemplates()) {
            if (template.match(matchTemplateName, pathAsString, fieldName, matchType)) {
                return template;
            }
        }
        if (matchTemplateName != null) {
            throw new DocumentParsingException(
                parser().getTokenLocation(),
                "Can't find dynamic template for dynamic template name [" + matchTemplateName + "] of field [" + pathAsString + "]"
            );
        }
        return null;
    }
    
    
    private static class CopyToParser extends FilterXContentParserWrapper {
        enum State {
            FIELD,
            VALUE