[Design] - Prototype of ResourceModel

/cc @yishaigalatzer @harshgMSFT 

Not included here:
- Model Metadata integration
- Sample generation interface
- Filter interface for customizing formatters

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.
The 'first' (closest to the action) attribute to set ContentTypes wins