Make omekatopkg generate a yaml with a thumbnail for the package.
Add an `--icon` option to specify a icon path to omekatopkg. The icon file is read and the content is added to (catalog) metadata as base64 string. The test file 'an-image.jpg` is now a three zero bytes. (bytes([0, 0, 0])). It is used to be sure than the icon is read correctly and transformed correctly ('AAAA' in base64)