VC logo and background image of credential: must be stored

[hawkbee1]

Currently we cache those images when it comes from an url. We need to keep this image in the metadata of the credentials so that the images stay if the issuer change or disappear. Furthermore those images are part of the backup which would be automatic if stored correctly in the metadata. Since VC metadata are stored in secure storage it will probably increase significantly the secure storage size :-/