Implement serial control of velocity and throttle

[samderanova]

As part of #37, allow continually outputting the PDM waveform with the last known parameters rather than just a single period. Using multicore functionality, a separate core monitors the serial input and updates the LIM control message through a mutex lock. We still need to implement expiration time for safety reasons.

Changes

• Create a global variable to store the velocity and throttle
  • Add a mutex to prevent concurrent access to it
• Utilize Raspberry Pi Pico's second core to run a function to read input and update the global variable

[taesungh]

Good checkpoint for now, nice to see we can sustain the parameters for continual output and update them on the fly. Will still need to implement the expiration time.

[taesungh]

Looks good, maybe in the future we could also consider implementing our own coupled Mutex to avoid having the data and the lock separate.