Introduce a new RequestHandler base class to introduce a shared session,
centralize HTTP request management and error handling across plugins.
Key changes:
- Add RequestHandler base class with a shared/cached session
- Convert TimeoutSession to use SingletonMeta for proper resource
management
- Create LyricsRequestHandler subclass with lyrics-specific error
handling
- Update MusicBrainzAPI to inherit from RequestHandler
- Instead of checking for empty `artist` query, use `va_likely`
parameter to determine whether we should query for Various Artists or
not.
- `album` / `title` is always a truthy string - no need to handle empty
criteria case
- `tracks` list always has at least one track - no need to check for
`len(items)`
