feat: Spotify support

[aquelemiguel]

Closes #50.

Aims to integrate with already existing code for YouTube playback. Under the hood, it fetches metadata from the Spotify link, builds a relevant query and feeds it to yt-dlp search function.

Tasks

• [x] Reply with warning if environment variables are missing
• [x] Minimize unwraps in spotify.rs
• [x] Investigate alternatives to lazy_static! and simplify ClientError init
• [x] Reply with warning if user submits a private playlist
• [x] Reply with warning if user submits an invalid Spotify link
• [x] Add pkg-config back to the Dockerfile

Screenshots

Success

Failed to authenticate with Spotify

Failed to fetch track/album/playlist

Failed to fetch private/algorithm playlist

[joao-conde]

Tag me back for review when your tasks are completed