Introduced PreserveReferencesAttribute

[Horusiath]

At this moment Hyperion defines an ability to preserve (and optimize serialization of) references for all reference types at a serializer level. There's no way to tell "I want to use this optimization, but only for objects of given type", which could be useful for things like i.e. Akka.NET IActorRef (if we'll use serializer sessions properly).

This PR introduces PreserveReferencesAttribute, which can be marked on class level. All objects with this attribute will override default SerializerOptions.PreserveObjectReferences settings.

[Aaronontheweb]

@Horusiath needs a rebase

[Danthar]

Besides rebase. Could also use some docs. Like a simple paragraph explaining what it does and why you would want to use it.