Add credential arguments to `check_credentials()`

[arthur-shaw]

Currently, the function silently, and without user override, loads credentials from .Renviron.

However, one may want to provide credentials for several reasons:

• Try/check credentials without having those credentials set
• Check credentials in a Shiny app without affecting the user's environment (i.e., writing to .Renviron)

Note: this might require changes to other functions where check_credentials() is part of the body and implementation logic.