Lay groundwork for new Spotify API before removing `spirc`
* Add token provider
* Introduce HTTP client
* Introduce caching `ApResolver` component
* Remove `keymaster` and update example
* Use `PacketType` instead of hex identifiers
* Document new unknown packet 0xb6
Re-add default impl to SessionConfig and make Credentials::with_password generic over Into<String>
add docs for Credential
reintroduce old Default impl for SessionConfig
use the third argument for the track-to-play rather than a testing id
