Add test for updating embedded relation using plain identifier

[teohhanhui]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#3133
License	MIT
Doc PR	N/A

Targeting 2.4 so that we're able to demonstrate that it works correctly on 2.4.

Background:

Before #3036, the $options we pass to the PropertyNameCollectionFactory was wrong, because it was using the (Serializer) $context directly. Specifically, the $context['groups'] were not translated to $options['serializer_groups'], so they had no effect. (There could have been many bugs caused by that.)

That bug fix however in turn revealed the problem with incorrect property metadata set on API resources in some projects (see for example #3133).

For example, the id property in the related resource is not in the serializer groups used for denormalization, which makes sense at first glance, because you don't want to allow updating the id. However, the ItemNormalizer uses the property metadata to identify the correct identifier property, so it's actually required that the id property must be part of the serializer groups used for denormalization (only applicable in the case of using plain identifiers, where you're sending the id property). To disallow updating the id, the correct solution is to use @ApiProperty(writable=false).