Add IsolatedStorageFile.GetEnumerator()

[JeremyKuhne]

IsolatedStorageFile.GetEnumerator() allows you to enumerate all stores for the current user.

How to: Enumerate Stores for Isolated Storage

[JeremyKuhne]

Subtask of #17629

[JeremyKuhne]

Don't currently plan to implement this as it depends on serialization of System.Security objects.

[JeremyKuhne]

@weshaggard When you iterate existing stores you need to expose IsolatedStorage.*Identity which is serialized/deserialized from disk. Not really plausible without Security related classes- not sure how we want to reflect that in NetStandard decisions.

[Petermarcu]

Is this in netstandard 2.0 or a future milestone. We shouldn't have future and netstandard2.0 on any issues.

[weshaggard]

@JeremyKuhne I would just have this return known stores, or empty. I don't think we need to worry about the full enumeration that we do on desktop.

[JeremyKuhne]

@weshaggard I currently have it as PlatformNotSupported. There is no way to return known stores without serializing our identity types, which I didn't want to do as it would collide with desktop. I'm currently relying on the fact that desktop will serialize it's own identity for the given store- and we don't have access to those types.

[JeremyKuhne]

dotnet/corefx#12641 to track making this return empty, closing the more complicated scenario pending user feedback.