Add Generics support

[toddbaert]

Support for generics. This is pretty broad, but there are two parts to this: generics for internal use in the SDK, and generics exposed to provider authors and application authors. We will want to be careful about the approach we take in order to make the transition from v1 to v2 as seamless as possible.

We want to add generics support to the SDK. We need some consensus on exactly how much we want to expose to users. Potential places where we can use generics:

• Resolution details
• Evaluation details
• Hooks
  ...