Fix Authentication by handing it over to garth

[flyingflo]

For #113

Use garth for auth and api

Replaces the login dance and api calls, and fixes the broken login.

Tokens are long-lived and can be stored, thus username and password are optional.

TODO:

• Token storage options
• Tests
• Cleanup

[petergardfjall]

Thanks for showing an interest in the tool.

I already turned down a PR to integrate with garth, and I still feel the same way about using a library that downloads credentials (whose?) from some S3 bucket. (see https://github.com/petergardfjall/garminexport/pull/104#issuecomment-1806817872 for more details).

I might reconsider but I would prefer another way, if possible.