IOptionsSnapshot should support scoped IConfigureOptions (ie. DbContext)

[HaoK]

Consuming a DbContext to configure options per request should be possible via.

   public class DbConfigure : IConfigureServices<MyOptions> {
       public DbConfigure(DbContext context) { Context = context; }
       public Configure(MyOptions options) {
           UseContextToConfigure(options);
       }
   }

   services.AddScoped<IConfigureService<MyOptions>, DbConfigure>();

This doesn't work today because we are caching the options until a change evicts (which works for the singleton IOptions but blocks this scenario)

cc @divega @glennc

[HaoK]

c68ede3