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
assertNotNull(doc.rootDoc().getField(IdFieldMapper.NAME));
    }
    public void testDotsWithFieldDisabled() throws IOException {
        DocumentMapper mapper = createDocumentMapper(fieldMapping(b -> b.field("enabled", false)));
        {
            ParsedDocument doc = mapper.parse(source(b -> {
                b.field("field.bar", "string value");
                b.field("blub", 222);
            }));
            assertNull(doc.rootDoc().getField("field"));
            assertNull(doc.rootDoc().getField("bar"));
            assertNull(doc.rootDoc().getField("field.bar"));
            assertNotNull(doc.rootDoc().getField("blub"));
        }
        {
            ParsedDocument doc = mapper.parse(source(b -> b.field("field.bar", 111)));
            assertNull(doc.rootDoc().getField("field"));
            assertNull(doc.rootDoc().getField("bar"));
            assertNull(doc.rootDoc().getField("field.bar"));
        }
        {
            ParsedDocument doc = mapper.parse(source(b -> b.field("field.bar", new int[] { 1, 2, 3 })));
            assertNull(doc.rootDoc().getField("field"));
            assertNull(doc.rootDoc().getField("bar"));
            assertNull(doc.rootDoc().getField("field.bar"));
        }
        {
            ParsedDocument doc = mapper.parse(source(b -> b.field("field.bar", Collections.singletonMap("key", "value"))));
            assertNull(doc.rootDoc().getField("field"));
            assertNull(doc.rootDoc().getField("bar"));