[Design] Refactoring the ControllerActionDescriptorProvider

This change will remove IActionDiscoveryConventions, and merges the functionality contained therein into a new extensibility point that discovers and creates `ControllerModel` instances.

TODO: Test cleanup, docs, deleting dead code

**Please give feedback on**
- Naming of new classes methods
- Breakdown of functionality/extensibility in DefaultControllerModelBuilder
new extensibility point. Replaces `IActionDiscoveryConventions`