Add option for users to bypass gen_auth_get() in order to use it with CI/CD

[yannikbuhl]

Right now, the possibilities for automation with the package are a little limited since it requires the user to interactively save the credentials in a secure .rds file from which it retrieves the credentials upon API call with a key.

This, however, leads to the problem that using the package in pipelines such as GitHub actions is difficult. This is why one could introduce a possibility for the user to bypass storing credentials securely on purpose and, for example, specify them directly in the function call. This would allow users to, e.g., use their API tokens in an automated fashion.

This should be accompanied with sufficient security warnings.

[yannikbuhl]

This will be done by implementing issue #60.