Officially support URL signing keys that handle multiple URL prefixes
The original URL signing implementation allowed multiple URL prefixes to be handled by the same URL signing key. This was, however, not an officially supported feature and it was removed in Opencast 7.0.
The goal of this task is to officially allow URL signing keys to support multiple URL prefixes.
This is mainly a convenience function (you could use different keys using the same secret to achieve the same), but it also keeps the number of keys that need to be tracked by the verifiers smaller.