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
b.startObject("foo.bar");
            {
                b.field("baz", 789);
            }
            b.endObject();
        }));
        assertNull(doc.dynamicMappingsUpdate()); 
        List<IndexableField> fields = doc.rootDoc().getFields("foo.bar.baz");
        assertEquals(3, fields.size());
        assertEquals("IntField <foo.bar.baz:123>", fields.get(0).toString());
        assertEquals("IntField <foo.bar.baz:456>", fields.get(1).toString());
        assertEquals("IntField <foo.bar.baz:789>", fields.get(2).toString());
    }
    public void testDotsWithExistingNestedMapper() throws Exception {
        DocumentMapper mapper = createDocumentMapper(fieldMapping(b -> {
            b.field("type", "nested");
            b.startObject("properties");
            {
                b.startObject("bar").field("type", "integer").endObject();
            }
            b.endObject();
        }));
        ParsedDocument doc = mapper.parse(source(b -> b.field("field.bar", 123)));
        assertEquals(123, doc.docs().get(0).getNumericValue("field.bar"));
    }
    public void testUnexpectedFieldMappingType() throws Exception {
        DocumentMapper mapper = createDocumentMapper(mapping(b -> {
            b.startObject("foo").field("type", "long").endObject();
            b.startObject("bar").field("type", "boolean").endObject();
        }));