feature: Implement Etsy Oauth

[tyler71]

Similar to existing oauth2 implementations

https://developer.etsy.com/documentation/essentials/authentication/

Example query:

    https://www.etsy.com/oauth/connect?
      response_type=code
      &redirect_uri=https://<YOUR_INSTANCE>/oauth/callback/etsy
      &scope=transactions_r%20transactions_w
      &client_id=1aa2bb33c44d55eeeeee6fff&state=superstate
      &code_challenge=DSWlW2Abh-cf8CeLL8-g3hQ2WQyYdKyiu83u_s7nRhI
      &code_challenge_method=S256

This is required for using the shop receipts api endpoint (when a order comes in) as well as other functionality.

After requesting an app and being approved, to edit callback url:
https://www.etsy.com/developers/ -> Manage your apps -> (Click on app name) -> Edit your callback URLs -> Add callback URL