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.field("store", false);
                            b.field("doc_values", false);
                        }
                        b.endObject();
                    }
                    b.endObject();
                }
                b.endObject();
            }
            b.endArray();
        }));
        String field = randomFrom("foo", "foo.bar", "foo.bar.baz");
        ParsedDocument doc = mapper.parse(source("1", b -> b.field(field, "true"), null, Map.of(field, "booleans")));
        List<IndexableField> fields = doc.rootDoc().getFields(field);
        assertThat(fields, hasSize(1));
        assertThat(fields.get(0).fieldType(), sameInstance(StringField.TYPE_NOT_STORED));
        DocumentParsingException error = expectThrows(
            DocumentParsingException.class,
            () -> mapper.parse(source("1", b -> b.field(field, "hello"), null, Map.of(field, "foo_bar")))
        );
        assertThat(
            error.getMessage(),
            containsString("Can't find dynamic template for dynamic template name [foo_bar] of field [" + field + "]")
        );
    }
    public void testWrongTypeDynamicTemplate() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> {
            b.startArray("dynamic_templates");
            {
                b.startObject();