Migrate to Open-Source Tokens

[MikeBishop]

Tesla has just added the option of Open-Source Tokens to the fleet API, where applications can get client IDs that do not have an associated client secret. That's exactly what we need here, and relieves customers of running their own integration.

The migration looks pretty straightforward and might not even require code changes, but we should a) register TWCManager, b) validate that assumption, and c) update documentation.

[MikeBishop]

@ngardiner, do you want to do the registration and get TWCManager a token? We shouldn't duplicate.

[MikeBishop]

Hm, it appears you still have to provide fixed URLs in the registration. That may be a barrier we need to think through. Maybe we could embed the actual endpoint in the state value and run a server that 301s you back to the actual target? On second thought, I believe that server would potentially be able to get people's tokens. Though they could at least validate that it hadn't done so after the fact....

[dtiefnig]

The user auth process looks pretty straight forward and I can't see why it would need to interact with an additional service. Maybe the URLs can be filled with arbitrary values during registration and it would still work? Could easily try with a https://webhook.site/ URL or so and see if anything arrives there.

Found your thread with Tesla btw. and am linking it here as a reference for others.
teslamotors/vehicle-command#111