feat: Add email address and fingerprint to exported key file names (#5694)

[iequidoo]

This way it's clearer which key is which and also adding the key fingerprint to the file name avoids overwriting another previously exported key. I think this is better than adding an incremental number as we do for backups, there's no need to export a key several times to different files.

Also i decided not to add a date to file names, seems it's not as useful as for backups.

Close #5694