[Design] - Prototype of ResourceModel

/cc @yishaigalatzer @harshgMSFT 

Not included here:
- Model Metadata integration
- Sample generation interface
- Filter interface for customizing formatters
- Xml Documentation access

Where possible I made decisions that are similar to webapi api explorer. See comments in the code for context on specific decision points and open questions.
It's limited to `ReflectedActionDescriptor` because we only want to create resource representations for things we understand. If your actions behave totally differently then you should implement a provider.