Tether API + mm support

[avtolstoy]

Testing

https://www.notion.so/particle/6-2-0-Tethering-testing-118d40caa5dc807ea313de75973a049c?pvs=4

Test application

#include "application.h"

SerialLogHandler dbg(115200, LOG_LEVEL_ALL);

SYSTEM_MODE(SEMI_AUTOMATIC);

SYSTEM_THREAD(ENABLED);

/* executes once at startup */
void setup() {
    // waitUntil(Serial.isConnected);
    // Enable Cellular
    Cellular.on();
    Cellular.connect();
    // Bind Tether interface to Serial1 @ 921600 baudrate with default settings (8n1 + RTS/CTS flow control)
    Tether.bind(TetherSerialConfig().baudrate(921600).serial(Serial1));
    // Turn on Tether interface and bring it up
    Tether.on();
    Tether.connect();
}

void loop() {

}