Allow PIN caching for keys on smartcard

[ei-ke]

Is your feature request related to a problem? Please describe.

Caching the unlocked key is (of course) not working for keys that live on a smartcard.

Describe the solution you'd like

Please add an option to cache the PIN for the private PGP/GPG key stored on e. g. a YK like it's done in OpenKeychain and purge the cached key when the screen gets locked.

Describe alternatives you've considered

Combine it with the already available "Keep key in RAM until the background service is stopped" option, though I'd prefer the first option or the option to stop the background service when the screen is locked.

Additional context