[REFACTOR] introduces ApiRequest and dynamic dispatch across implementations

[withtwoemms]

Summary:

There are different ways to make a request. The convenience of the old PokeApiRequest is that it is allows for separation between the declaration of the request and its invocation. This general structure is captured by the new DeferredRequest protocol. The ApiRequest enum is introduced as an encapsulation of the various DeferredRequest implementations. Dynamic dispatch across implementations can be implicitly controlled by setting the "API_REQUEST_IMPL" environment variable.

Notes:

• implicit control of implementation dispatch was chosen as it's the simplest path towards runtime control of request behavior. In the future, consideration could be given to an explicit rendition which would necessitate changes in the function signatures of pokedex.api.client members and would relieve dependence on test mocks.