Generic file recovery and full FUSE read support

What's Changed

• Implement the recover CLI command to recursively restore files,
  directories, and symlinks from a BTRFS device or image.
• Map logical BTRFS extents to physical offsets on the device using
  Index.Physical() to copy file chunks, and handle sparse holes.
• Support recovering unreferenced subvolumes under subvol_<ID> folders.
• Fix the FUSE file reader in internal/rescuefs/files.go to support
  multiple extents, exact attributes from InodeItem, and offset overlap.
• Add macOS 12 runner using Go 1.17.13 by @cblichmann in #13

Full Changelog: v0.5...v0.7