feat(link statistics): Add Link Statistics API

[ZZ-Cat]

Overview

This Pull Request introduces the ability to read link statistics from the CRSF data stream, using an event-driven API.

Details

• In CFA_Config.hpp, CRSF_LINK_STATISTICS_ENABLED is set to 1 by default, thus enabling CRSF for Arduino's ability to read link statistics.
• CRSFforArduino::setLinkStatisticsCallback(serialReceiverLayer::link_statistics_t) is used to set your desired callback function for reading the incoming link statistics.

The following example highlights the new API...

#include "CRSFforArduino.hpp"

/* Declare a null pointer to CRSF for Arduino. */
CRSFforArduino *crsf = nullptr;

/* This needs to be up here, to prevent compiler warnings. */
void onLinkStatisticsUpdate(serialReceiverLayer::link_statistics_t);

void setup()
{
    Serial.begin(115200);
    while (!Serial)
    {
        delay(10);
    }

    Serial.println("Link Statistics Example.");

    /* Initialise CRSF for Arduino. */
    crsf = new CRSFforArduino();

    if (!crsf->begin())
    {
        Serial.println("CRSF for Arduino failed to initialise.");

        delete crsf;
        crsf = nullptr;

        while (1)
        {
            delay(10);
        }
    }

    /* Set your link statistics callback. */
    crsf->setLinkStatisticsCallback(onLinkStatisticsUpdate);

    /* Example firmware is ready. */
    Serial.println("Ready.");
    delay(1000);
}

void loop()
{
    /* Update CRSF for Arduino in the main loop. */
    crsf->update();
}

void onLinkStatisticsUpdate(serialReceiverLayer::link_statistics_t linkStatistics)
{
    /* Here is where you can read out the link statistics.
    You have access to the following data:
    - RSSI (dBm)
    - Link Quality (%)
    - Signal-to-Noise Ratio (dBm)
    - Transmitter Power (mW) */
    static unsigned long lastPrint = 0;
    if (millis() - lastPrint >= 200)
    {
        lastPrint = millis();
        Serial.print("Link Statistics: ");
        Serial.print("RSSI: ");
        Serial.print(linkStatistics.rssi);
        Serial.print(", Link Quality: ");
        Serial.print(linkStatistics.lqi);
        Serial.print(", Signal-to-Noise Ratio: ");
        Serial.print(linkStatistics.snr);
        Serial.print(", Transmitter Power: ");
        Serial.println(linkStatistics.tx_power);
    }
}

Additional notes

The Link Statistics Event only fires when complete CRSF packets are received.
ExpressLRS receivers emit several Link Statistics packets for about five seconds after the receiver has gone into Fail-Safe state.
This is reliable enough to see whether-or-not the receiver has lost connection with the transmitter.
By watching the Link Quality value, we can determine the stability of the connection, plus whether-or-not there's a Fail-Safe.