Add fitbit subscription

[blootsvoets]

Fitbit subscription endpoint for getting subscription info. Still missing:

• Notification storage
• Fetching notifications using the connector
• Subscribing users

WIP for #88. Feel free to drop a line and continue this line of work.

[yatharthranjan]

Do you think this will be better placed in https://github.com/RADAR-base/RADAR-PushEndpoint ? The garmin backfill is something similar (we make requests and keep offset in redis) so probably some of that could be reused for this too.

[blootsvoets]

Yes, good point. I've moved the data conversion into a separate radar-fitbit-converter library, so it can be used both in this package and later in the push endpoint package. Still needs package publishing gradle setup and Github actions. The endpoint code will remain for now, as a reference for implementation in the push endpoint.

[yatharthranjan]

Ok great, thanks @blootsvoets, @jzhou59 will be continuing this piece of work (as part of gsoc).