Start Yahoo Implementation #336

[caltonji]

Starting work on: https://github.com/ffverse/ffscrapr/issues/336

This PR implements ff_connect, ff_userleagues, and ff_franchises for Yahoo.

To login, users will need to

1. Obtain your league_id from any yahoo URL in your leagues. In this link it's 77275: https://football.fantasysports.yahoo.com/f1/77275/10
2. Obtain an auth token by signing into Yahoo here https://lemon-dune-0cd4b231e.azurestaticapps.net/.

I built that website for grabbing the auth token as part of a past project https://introductory.medium.com/download-yahoo-fantasy-football-data-bc1e1db7ee49

I plan to finish the Yahoo implementation, but I'm starting small.