ID3v2: Unify generic conversions

[Serial-ATA]

Tag::save_to() and Into::<Id3v2Tag>::into(tag) would take different paths, causing frames to not get merged (and possibly other weird behavior). Now, both take the same path.

All frames have been updated to use Cow, to avoid allocations in the Tag::save_to() path (unless absolutely necessary, like for track numbers).

This should be easier to maintain now that everything's unified. And possibly more efficient, since it's no longer cloning everything.

closes #349

[uklotzde]

After writing ItemKey::IntegerBpm as ID3v2 into an empty file no tags are written and the file remains empty.

let mut file = copy_named_temp_file("tests/assets/empty.mp3");

  {
      let tagged_file = Probe::new(&mut file.as_file())
          .guess_file_type()
          .unwrap()
          .read()
          .unwrap();
      assert_eq!(FileType::Mpeg, tagged_file.file_type());
      assert!(tagged_file.tags().is_empty());
  }

  let mut tag = Tag::new(TagType::Id3v2);
  tag.insert_text(
      ItemKey::IntegerBpm,
     122.to_string(),
  );
  tag.save_to_path(file.path(), WriteOptions::default())
      .unwrap();

[uklotzde]

https://github.com/aoide-org/aoide-rs/actions/runs/19388551698/job/55478819021

[Serial-ATA]

@uklotzde Good catch, fixed in #579.