NewFileStore returns a concrete implementation of a Store that is based on storing the cert/key pairs in a single file per pair on disk in the designated directory.

NewManager returns a new certificate manager.

Config is the set of configuration parameters available for a new Manager.

Gauge will record the remaining lifetime of the certificate each time it is updated.

Manager maintains and updates the certificates in use by this certificate manager.

Store is responsible for getting and updating the current certificate.

NoCertKeyError indicates there is no cert/key currently available.