Add ability to specify whether options of IOptionsProvider should be part of solution snapshot

[tmat]

Currently we decide based on the assembly the options provider is in:

// We only consider the options defined in the the DefaultAssemblies (Workspaces and Features) as serializable.
// This is due to the fact that other layers above are VS specific and do not execute in OOP.
if (!MefHostServices.IsDefaultAssembly(provider.GetType().Assembly))

http://sourceroslyn.io/#Microsoft.CodeAnalysis.Workspaces/Options/GlobalOptionService.cs,66

It would be better to explicitly declare where the option should be available via an argument to ExportOptionProvider. E.g. ClientOnly = true would mean the option is not serialized to the solution snapshot.

[tmat]

IOptionsProvider are not needed anymore.