* Add hardware key service interface; Add mock service for tests.
* Add piv implementation of hardware key service.
* Remove old yubikey parsing logic in favor of hardwarekey.PrivateKeyRef.
* Remove HardwareSigner in favor of hardwarekey.PrivateKey.
* Add test; Cleanup comment; Don't require pin/touch prompts in tests with MockHarwdareKeyService by default.
* Fix tests.
* Add TODOs.
* Use unhashed digest for WarmupHardwareKey.
* Address comments.
* Fix race condition in test service.
* Address comments.