fix(auth): add support for setting auth header and cookie manually

[JakobLichterfeld]

An approach for mitigating issue petergardfjall#113. This PR allows the user to set the Authorization header and JWT_FGP cookie through command line args, e.g.,

export TOKEN="longer_token_here"
export JWT_FGP="shorter-cookie-here"
garmin-backup --token=$TOKEN --jwt_fgp=$JWT_FGP username

Where the token/cookie can be found under the Network>Headers/Cookies tab in your browser's DevTools in requests to connect.garmin.com.

petergardfjall#115