Port opensubtitles plugin to new API
opensubtitles currently uses the old opensubtitles API which used XML-RPC, implemented using the XML-RPC support in libsoup 2.4.
The XML-RPC support is gone in libsoup 3, and there's a new OpenSubtitles.org API, which we should use instead.
See: https://opensubtitles.stoplight.io/docs/opensubtitles-api/e3750fd63a100-getting-started
It might be possible to use librest to implement this.