related to T8291

When exporting a certificate via Okular's certificate viewer, it currently creates a file with the ending ".cer" and always opens the directory C:\Windows\System32 for saving, even if it was exported once before to the users Document folder:

The content is binary. The file can be imported without issues but ".cer" is used for S/MIME and we should offer either ".gpg" or better the default export format for Kleopatra, ".asc".

Let's use the same naming convention as Kleopatra does:

NAME_0xKey-ID_public.asc