Support for Garmin Connect API

[a0a7]

Goals

1. Add support for the Garmin Connect API in Auth.js

Non-Goals

No response

Background

Auth.js already supports one excercise tracking app--Strava--but the other major excercise tracking app, Garmin Connect, is not supported by Auth.js. If it were supported, auth.js would be a very attractive option for people looking to build OAuth applications in the domain of fitness. That Garmin Connect is missing is a major gap in the auth domains covered by auth.js, as exercise tracking & integration with such is a fairly major use case for OAuth.

Proposal

Garmin connect would be implemented like any other OAuth provider.