Add methods to reference content across different geographical regions & map explicit to clean-equivalent content.

[rudrankriyam]

Usage:

• Referencing content across different geographical regions. In the example below, the target storefront is "tw" for Taiwan:

let equivalentAlbum = try await album.equivalent(for: "tw")

let equivalentAlbums = try await albums.equivalents(for: "tw")

• Map explicit to clean-equivalent content:

  
  let cleanSong = try await song.clean

let cleanSongs = try await songs.clean

> Note: For both methods, you can only get the equivalents for songs, albums, and music videos.