feat: Allow to use gopass instead of plain pass

[poettig]

This merge request allows it to switch to using gopass instead of plain pass.
It removes the hard dependency on the pass executable - because that would not allow the plugin to work on systems without pass, but with gopass. Instead, it shows an error action at runtime.

[okaestne]

Do you agree that it might be even more useful to make the executable path configurable instead?