Add support for OAuth authentication strategy

[0xf4b1]

Spotify deprecated authentication with username and password and this authentication strategy no longer works in librespot-java resulting in BadCredentials exception (#921).
This PR adds OAuth as new authentication strategy as replacement. It's basically a backport of the PR librespot-org/librespot#1309 from the related librespot project.

It adds all the OAuth logic as a new class in the lib module which is capable of providing LoginCredentials of type AUTHENTICATION_SPOTIFY_TOKEN by following the Authorization Code with PKCE Flow.
The session with token authentication can not be used with the keymaster which is why it does a reconnect with the ReusableAuthCredentials, same as in librespot.
To avoid having to authenticate with OAuth with every single run, it is intended to be used together with having storeCredentials enabled and it will default to use the stored credentials if they exist.

The OAuth class provides the full authentication flow with the flow method including running a simple callback server suitable for a local terminal-based environments which can be used as reference and the player module and FileConfiguration is also adjusted to support it.

The required steps for OAuth can also be done manually which makes it possible to integrate into non-terminal-based environments, such as Android in my case, in the following way:

OAuth oauth = new OAuth("<client_id>", "<redirect_url>");
String url = oauth.getAuthUrl();
// Let user visit the url and retrieve the code request parameter of the callback
oauth.setCode(code);
oauth.requestToken();
Session.Builder(conf).credentials(oauth.getCredentials()).build()

Thanks a lot for the library! I know it is no longer actively maintained but I hope we can keep it working :)

Fixes #921

[0xf4b1]

Hey @devgianlu, could you have a look and do a quick review when you find a moment?

[devgianlu]

Thank you for the PR!

[maxwen]

Awesome - thx

[0xf4b1]

Thanks for review and merging @devgianlu! I wonder if you would mind creating a new release including this PR so that users can continue using the library without having to build themselves?

[devgianlu]

https://github.com/librespot-org/librespot-java/releases/tag/v1.6.4

[0xf4b1]

Awesome, thanks a lot again! :)