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
DocumentMapper mapper = createDocumentMapper(fieldMapping(b -> b.field("type", "object")));
        ParsedDocument doc = mapper.parse(source(b -> b.startArray("field.bar.baz").value(0).value(1).endArray()));
        assertEquals(2, doc.rootDoc().getFields("field.bar.baz").size());
        Mapper fooMapper = doc.dynamicMappingsUpdate().getRoot().getMapper("field");
        assertNotNull(fooMapper);
        assertThat(fooMapper, instanceOf(ObjectMapper.class));
        Mapper barMapper = ((ObjectMapper) fooMapper).getMapper("bar");
        assertNotNull(barMapper);
        assertThat(barMapper, instanceOf(ObjectMapper.class));
        Mapper bazMapper = ((ObjectMapper) barMapper).getMapper("baz");
        assertNotNull(bazMapper);
        assertThat(bazMapper, instanceOf(NumberFieldMapper.class));
    }
    public void testDynamicDottedFieldNameLongArrayWithExistingParentWrongType() throws Exception {
        DocumentMapper mapper = createDocumentMapper(fieldMapping(b -> b.field("type", "long")));
        DocumentParsingException exception = expectThrows(
            DocumentParsingException.class,
            () -> mapper.parse(source(b -> b.startArray("field.bar.baz").value(0).value(1).endArray()))
        );
        assertThat(exception.getMessage(), containsString("failed to parse field [field] of type [long]"));
    }
    public void testDynamicFalseDottedFieldNameLongArray() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> b.field("dynamic", "false")));
        ParsedDocument doc = mapper.parse(source(b -> b.startArray("foo.bar.baz").value(0).value(1).endArray()));
        assertEquals(0, doc.rootDoc().getFields("foo.bar.baz").size());
    }
    public void testDynamicStrictDottedFieldNameLongArray() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> b.field("dynamic", "strict")));
        DocumentParsingException exception = expectThrows(
            DocumentParsingException.class,