The `name` parameter of `[PersistentState]` should be made optional

[aradalvand]

I'm struggling to understand why a name necessarily has to be supplied when using [PersistentState].
The IGrainStorage implementation, in the ordinary case, would look at the type of the state in order to figure out how to resolve it. So, fundamentally, whether or not it's actually needed depends on the specific IGrainStorage implementation in use.

Therefore, there is no reason to force users to always supply a name at the API level, when, at least in many cases, it may not be necessary.