Adds desktop authentication flow

[tayden]

Adds the desktop login flow for more seamless API authorization in desktop apps. Users will no longer be required to copy and paste credentials.

This needs more work, but is functional as is.

[tayden]

To do:

• Refactoring/simplification of Client.py for easier maintainability
  • Split the desktop client out into a new DesktopClient class? Or keep things integrated?
  • Use a strategy pattern and keep the client class as a factory class to maintain backwards compatability.
• Opt-in or opt-out of token refresh
• Incorporate some of the quality-of-life improvements that the R client has (like saving the base url) [probably in a new PR]

[tayden]

This is good to go! It's pretty slick.

• You can select the "desktop" or "web" (default) auth flow when you initialize the class.
• There's a initialization flag to disable use of the refresh tokens (currently, does use refresh tokens by default)
• I also added the ability to use relative API urls (e.g. /whoami vs f"{api.root_url}/whoami").
• Code is setup to add additional auth flows in the future if needed (e.g. a "Service" flow, perhaps?)
• Configurable credentials file location
• Configurable user agent identifier

[boshek]

This looks great. I tried it out locally at it worked perfectly. And the code is rock solid. What a great improvement. One non-blocking thought here is whether we should start including a changelog or release notes here.

thanks for the all tests too!

[boshek]

I believe this also closes #26