Split the AccessTokenTrait::convertToJWT() function so that the token can be customized

[rhertogh]

Split the \League\OAuth2\Server\Entities\Traits\AccessTokenTrait::convertToJWT() function into two separate functions so that implementing classes can customize the token. E.g.

class AccessToken implements AccessTokenEntityInterface
{
// ...
    use AccessTokenTrait {
        getJwtBuilder as traitGetJwtBuilder;
    }
// ...
    private function getJwtBuilder()
    {
        $builder = $this->traitGetJwtBuilder();

        $builder->withClaim('my_custom_claim', 'custom claim value');

        return $builder;
    }

[parallels999]

#1328

[rhertogh]

Closed in favor of #1328

[parallels999]

Also, leave your comment on that PR, so the authors review it at least