[$1000 bounty +community] Rivian: read set speed

Rivian longitudinal control is ready to go... we just can't read the set speed or the button presses from the car.

This bounty is for something that can ship to openpilot release, so should be the same harness, no FW flashing, etc. We can consider a "sub bounty" for something that's alpha-long quality, but really we want this to ship to release.

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EDIT: the communtiy has organized a [GoFundMe](https://www.gofundme.com/f/help-solve-rivians-setspeed-issue-with-openpilot?attribution_id=sl:0bff65bb-d878-47e6-88c6-ad1e02cb2140&lang=en_US&utm_campaign=man_sharesheet_dash&utm_content=amp13_c-amp14_t1&utm_medium=customer&utm_source=copy_link) for additional incentive on the project.

No file or test is identified, so begin by locating the Rivian CAN/DBC handling in opendbc and reviewing how vehicle signals are exposed. Done means reading the set speed and button presses using the existing harness, without firmware flashing, at a quality suitable for an openpilot release.

Written by the indexing model from the issue text.