Add a function to store EU Digital COVID certificates

[aurorasmiles]

Now that Austria has fully enrolled in the EU Digital COVID certificates ("Grüner Pass"), it would be a nice feature to be able to store your digital certificate within the app. In theory, it would be enough to store the QR-code locally on the device, however, it would be nice to also display some basic information about the certificate. For comparison, the german "Corona Warn App" (as well as the independent https://github.com/Digitaler-Impfnachweis/covpass-android) from the RKI does already have this feature, and it provides the name, DOB, the validity, and of course the QR code.