Add common API for constructing device instances, improve introspection

[rytilahti]

New class methods in Device:

• all_supported_models() to return a dictionary model=>implementation class
• class_for_model(model: str) to return implementation class for the given model
• create(host: str, token: str, model: Optional[str]) to create a device instance

New cli commands:

• miiocli supported-models will print a list of supported models

TBD:

• [x] Tests
• [ ] Extract the factory into its own class
• [ ] Update README and docs for changes

Fixes #1117