Support for authorization_code in gem

[Lars Vonk]

The 5.0.0 version of the gem currently hardcodes the client credentials flow. It should support the https://developer.jortt.nl/#authorization-code-grant-type as well.

See

jortt-ruby/lib/jortt/client.rb

Line 42 in 16b9221

 @token = client.client_credentials.get_token(scope: "invoices:read invoices:write customers:read customers:write") 

[Lars Vonk]

Fixed in #34