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/test/java/org/elasticsearch/index/mapper/DocumentParserTests.java
builder.startObject("properties");
        builder.startObject("field").field("type", "keyword").endObject();
        builder.endObject().endObject().endObject();
        DocumentMapper mapper = createDocumentMapper(builder);
        ParsedDocument doc = mapper.parse(source(b -> b.field("field", "value")));
        
        assertNull(doc.dynamicMappingsUpdate());
        assertNotNull(doc.rootDoc().getField("field"));
    }
    public void testParseWithRuntimeFieldDottedNameDisabledObject() throws Exception {
        XContentBuilder builder = XContentFactory.jsonBuilder().startObject().startObject("_doc");
        builder.startObject("runtime");
        builder.startObject("path1.path2.path3.field").field("type", "keyword").endObject();
        builder.endObject();
        builder.startObject("properties");
        builder.startObject("path1").field("type", "object").field("enabled", false).endObject();
        builder.endObject().endObject().endObject();
        MapperService mapperService = createMapperService(builder);
        ParsedDocument doc = mapperService.documentMapper().parse(source(b -> {
            b.startObject("path1").startObject("path2").startObject("path3");
            b.field("field", "value");
            b.endObject().endObject().endObject();
        }));
        assertNull(doc.dynamicMappingsUpdate());
        assertNull(doc.rootDoc().getField("path1.path2.path3.field"));
    }
    public void testParseWithShadowedSubField() throws Exception {
        XContentBuilder builder = XContentFactory.jsonBuilder().startObject().startObject("_doc");
        builder.startObject("runtime");
        builder.startObject("field.keyword").field("type", "keyword").endObject();