User library & follow endpoint implementations

[KRTirtho]

Added support for following endpoints

Endpoints	Class	Method	HTTP method
/v1/me/following/contains	Me	isFollowing	GET
/v1/me/following	Me	follow	PUT
/v1/me/following	Me	unfollow	DELETE
/v1/me/albums	Me	savedAlbums	GET
/v1/me/albums	Me	saveAlbums	PUT
/v1/me/albums	Me	removeAlbums	DELETE
/v1/me/albums/contains	Me	isSavedAlbums	GET
/v1/playlists/{playlist-id}/followers/contains	Playlists	followedBy	GET
/v1/me/tracks	TracksMe	remove, removeOne	DELETE

Other smaller tweaks/bugfixes:

• Replaced non-required mutable var variables with immutable finals
• Future<Null> return types replaced with Future<void>
• TracksMe.contains throws a TypeError while casting List<bool>
• Tests created for newly added endpoints which are testable

Note: The FollowingType.user wasn't tested for Me.follow/Me.unfollow methods

Awesome library dude. Without spotify-dart, spotube wouldn't be possible. Keep up your awesome work❣️💪🏻

[KRTirtho]

@rinukkusu can I get some update/feedback/review regarding this PR if possible?

[hayribakici]

@KRTirtho it would be nice, if you mark the implemented endpoints in the wiki.

[KRTirtho]

Can I do that? I don't think I've permissions to edit wiki of a repo which isn't owned by me. But these ones should be checked

Follow API

• [x] Check If User Follows Artists or Users
• [x] Check if Users Follow a Playlist
• [x] Follow Artists or Users
• [x] Unfollow Artists or Users

Library API

• [x] Get User's Saved Albums
• [x] Save Albums for Current User
• [x] Remove Albums for Current User
• [x] Check User's Saved Albums
• [x] Remove User's Saved Tracks
• [x] Get User's Saved Tracks
• [x] Save Tracks for User

[hayribakici]

Cool, once this is merged, I'll check them for you.

[KRTirtho]

I also added a Bugfix causing onCredentialsRefreshed not getting called when its supposed to. (https://github.com/rinukkusu/spotify-dart/issues/123)

Reasons & explanations: https://github.com/rinukkusu/spotify-dart/issues/123#issuecomment-1138231777

[rinukkusu]

Hmmm, I also saw that you added some stuff, that older PRs also wanted to add, like the removeOne on TracksMe. My fault for not noticing that and having not a lot of time to process all of this, but generally not that great.