🔨 Move initializer script to a helper method on Pcloud::Client

[jhunschejones]

Description

To set up the token for OAuth2, users are currently required to download and run a script. This works well enough, but I'd like to expose this as a method on Pcloud::Client such that a user can just load an IRB or Rails console, require the gem, and call the method to start the setup process. I believe this will provide a smoother onboarding process than requiring users to download and run a separate script for setup.