Allow asymmetric objects to contain links to non-embedded objects

[nielsenko]

Currently having links to regular realm objects is not allowed in asymmetric objects. Trying to define a realm model like this

@RealmModel(ObjectType.asymmetricObject)
class _Asymmetric {
  _Symmetric? symmetric;
  // ...
}

@RealmModel()
class _Symmetric {
  // ...
}

is disallowed by realm-core. Opening a realm with such schema will throw:

  RealmException: Error opening realm at path <redacted>. Error code: 2016 . Message: Schema validation failed due to the following errors:
  - Asymmetric table with property 'Asymmetric.symmetric' of type 'object' cannot have a non-embedded object type.

But it is trivial to work-around this restriction by injecting an embedded object like this:

@RealmModel(ObjectType.asymmetricObject)
class _Asymmetric {
  late List<_Embedded> embeddedObjects;
  // ...
}

@RealmModel(ObjectType.embeddedObject)
class _Embedded {
  late RealmValue any;
  _Symmetric? symmetric;
}

@RealmModel()
class _Symmetric {
  // ...
}

From discussion with BAAS team they don't have any problems with supporting links to non-embedded objects, so I request we lift the restriction in realm-core as well, since it is not really enforced anyway.