First pass over the Provider interfaces, contracts, and related changes.
From callback-to-async. HttpAsync is nothing more than a Stub for now. It should be replaced with more efficient, and, meaningful code later on (nudge TinyHttp project for that).
Change the Factory class, as it is no longer creating anything. Instead, the EnableApproximation name has been chosen to indicate chapter number approximation functionality.
Add the missing IProvider.
Nullable chapter and volumes.
Code contracts for provider