* Added ViewComponentFeture and ViewComponentFeatureProvider to perform view component discovery.
* Changed view component discovery to use application parts.
* Changed ViewComponentDescriptorProvider to make use of Application parts.
* Added AddViewComponentsAsServices method on IMvcBuilder that performs view component
discovery through the ApplicationPartManager and registers those view components as
services in the service collection. Assemblies should be added to the ApplicationPartManager
in order to discover view components in them in them.
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