Add Container.Collection.RegisterConditional

[dotnetjunkie]

Allow collections to be registered conditional, similar to what Container.RegisterConditional does for non-collection registrations.

e.g.:

container.Collection.RegisterConditional(
    serviceType: typeof(ILogger),
    serviceTypes: new[] { typeof(SqlLogger), typeof(FileLogger) },
    c => c.Consumer.Target.Name.StartsWith("realLoggers"));

container.Collection.RegisterConditional(
    serviceType: typeof(ILogger),
    serviceTypes: new[] { typeof(ConsoleLogger), typeof(NullLogger) },
    c => !c.Handled);

The difficulty in this feature is the following:

• It should work for both ambient and flowing scope scenarios
• It should work for all supported collection types

Related to #1009 and #1010.

[dotnetjunkie]

hmmm... this is actually really tricky because the collection itself is a singleton stream. But having conditional elements means that multiple singletons must be present, where they differ in their elements. Perhaps we need a different design here.