[REQ][Slim4] Add OAuth2 server library

Created by: ybelenko

Is your feature request related to a problem? Please describe.

After spending too much time with own custom OAuth2 server implementations I've tried to find something recommended by framework authors. Here is list of server libraries(source Server Libraries):

• PHP OAuth2 Server and Demo
• PHP OAuth 2.0 Auth and Resource Server and Demo
• PHP OAuth2.0 for Silex and Demo
• PHP OAuth2.0 for Symfony and Demo
• Nette OAuth2 provider for Nette framework and Nette REST API bundle

Describe the solution you'd like

I'm going to add auth library from list above to PHP Slim server generator. I'm not sure which one yet, so feel free to suggest. I've already checked PHP OAuth2 Server and not satisfied, repo looks pretty outdated, there are many open issues and PRs. The most weird think is that author stores sha1 encrypted passwords in database by default instead of use of password_hash PHP builtin function.

Ref to described function: https://github.com/bshaffer/oauth2-server-php/blob/5a0c8000d4763b276919e2106f54eddda6bc50fa/src/OAuth2/Storage/Pdo.php#L437-L441

Additional context

If you have any experience with library from list above, please describe it. Keep in mind that PHP Slim Framework is very tiny and doesn't have hundreds of builtin helpers/utilities like Symfony has.

cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh @renepardon