Add control message expiration time

[taesungh]

Following from #24/#36, the inverter will output a single period of the PDM waveform for each control message. To have a consistent flow of power, this would require the pod's FSM to be synchronized with the AC frequency, but this is not known by the pod. To avoid overfilling a buffer of messages, we can set an expiration time on the control message and assume the same parameters (velocity and throttle) for subsequent periods until the previous message has expired at which point the inverter must wait for another message.

• Explore options to make the serial read non-blocking (async or threads)
• Record the time when a control message is received
• Refactor the read_control_message function to provide the previous known command if it has not expired
• Important: if a new control message is being sent, it should be used immediately

[taesungh]

Update: the Pico is running bare metal (no OS), so the arm-none-eabi compiler doesn't support threads. Sam instead suggested we look into the Pico's multicore capabilities.

[taesungh]

Functionality to sustain parameters and adjust on the fly added in #39, looking into expiration time next.