Add functionality to read and parse serial data from pod control

[taesungh]

In conjunction with UCI-HyperXite/hx9-control-system#35, we'll need the Pi Pico to be able to receive and process command messages from pod control running on the main Raspberry Pi.

• Create a new function for operating the inverter which waits for a serial command and calls the function to output a PDM waveform based on the given control parameters (velocity and throttle)
• Parse serial data according to the schema established as part of [Pod] Create abstraction for driving inverter board hx9-control-system#35
  • We might also want to provide some acknowledgement message back to pod control
• We will need to decide whether to store the last received command or always wait for a new command
  • Forcing the inverter to wait for a command would be safer since the LIM would turn off if a connection fault occurs
  • However if the primary FSM tick rate is insufficient, we can also consider using an expiration time

[taesungh]

Note: we can start implementing the protocol together with the pod-operation side, but we won't be able to actually test the signal output until we finish #12 and #20.

[taesungh]

I forgot that in #3, we originally planned for the inverter control to accept the speed of the pod. We'll need to change the control parameters from frequency and amplitude to speed and throttle.

[taesungh]

Amplitude control added in #29 and frequency relation added in #32. We'll also need throttle control from #33 before testing and might need a control parameter for phase depending on #31.