This commit introduces application parts as a concept on MVC.
An application part is an abstraction that allows you to expose some
feature or corncern in a way that is decoupled from their underlying source.
Examples of this include types in an assembly, emdeded resources, files on
disk etc.
Application parts are configured during startup by adding or removing them from
the application part manager available as part of IMvcBuilder and IMvcCoreBuilder.
The application part manager provides the ability to populate features from the
list of available application parts by using a list of application feature providers.
Application feature providers are responsible for populating a given feature given a
list of application parts.
Examples of application providers can be a ControllerFeatureProvider
that goes through the list of application parts, sees which one of those parts exposes types,
determines which of those types are controller types, and adds them to a ControllerFeature
that holds a list of all the types that will be considered controllers in the application.
