Implement partial library scans

[itsToggle]

As mentioned in #107 , refreshing large libraries can take quite a while. To combat this, a plex library scan can be limited to specific folders.

Partial library scans can sadly not be started via HTTP requests, they can only be called by the plex command line interface. This requires access to the plex excecutable, which is difficult to implement for different operating systems and setups. As @themegaphoenix mentioned, there is a project called AutoScan which can do this exact task and can be setup seperately. They offer a webhook interface, which plex_debrid can use to call for a partial library refresh.

This will be implemented as an optional feature along side the existing "HTTP" plex library refresh as part of a larger update regarding the implementation of library services. This larger update will make it possible to choose which libary service should be used to determine the current media collection, and which library services (!multiple) should be maintained by plex_debrid. This will allow you to refresh your plex libraries, aswell as add stuff you download to your trakt collection.