Migrate delegate callbacks to an observer pattern

Highlights

• API: Switched from delegate callbacks to an observer pattern (one-to-many). Implement FRadioPlayerObserver and call addObserver/removeObserver.
• Streams: Added support for non-live (file) streams with duration/currentTime updates, seeking, and more robust state handling.
• Packaging: SPM-only. Removed CocoaPods/Carthage files and Travis CI.
• Swift version: Bumped to Swift tools 5.5 (Xcode 13+).
• Demo app: Revamped SwiftUI-only demo (iOS 15+):
  • Reorganized into Views/Models/Resources; cleaner UI; uses AsyncImage for artwork.
  • Generated with XcodeGen (no .xcodeproj in repo).
• CI: Added GitHub Actions
  • spm.yml builds and tests the library.
  • demo.yml generates (XcodeGen) and builds the demo.

Migration notes (delegate → observers)

• Before (delegate, one-to-one):
  player.delegate = self
• Now (observers, one-to-many):
  player.addObserver(self) // conform to FRadioPlayerObserver
  // optionally: player.removeObserver(self)

Observer callbacks to implement

• radioPlayer(_:playerStateDidChange:)
• radioPlayer(_:playbackStateDidChange:)
• radioPlayer(_:itemDidChange:)
• radioPlayer(_:metadataDidChange:)
• radioPlayer(_:artworkDidChange:)
• radioPlayer(_:durationDidChange:)
• radioPlayer(_:playTimeDidChange:duration:)