Update issuer configuration cover image

Uploads the cover image of the default display of the issuer configuration. The platform stores the image and sets definitionDefaults.display.backgroundImage.uri to the public URL of the image. A client, for example the dashboard, can put it in a new credential definition; the create call itself takes the cover image of the organisation. Existing credential definitions do not change. The image must be PNG, JPEG, GIF or WebP, at most 5 MiB and 4096 x 4096 pixels (the default limits). An organisation has one issuer configuration, so the path has no id.