New 6-axis ACRO mode for Pixracer

Call for expert level FPV racing pilots: this PR: https://github.com/PX4/Firmware/pull/5372 makes ACRO mode use the full IMU solution for attitude, instead of just gyro feedback. It will be better (tighter lock-in at lower PID gain) as long as the IMU doesn’t get confused by continuous high acceleration maneuvers.

Anyone up to the challenge?

Original PR replaced by https://github.com/PX4/Firmware/pull/5498