Implementing sync since-last-upload feature

[fowie]

In v 3.0.2 I had implemented (on my own branch) a feature to get all activites from Peloton since the last Garmin sync. This would first query the Garmin API to get the DateTime of the latest workout, then get Peloton workouts after that DateTime, and submit them to Garmin. It worked great for quite a while, but looks like Garmin changed their API, so requests to https://connectapi.garmin.com/proxy/activity-service/activity/recent (or https://connectapi.garmin.com/proxy/activity-service-1.3/activity/recent) end up giving me a 400 Bad Request error. It seems like the application already has the ability to upload data into Garmin, is there an already-established way to grab the latest workout using the new sso.garmin.com API, or some other way?