Add a delay to mt_serial_init

[AnthonyErr]

Serial Connection Fix

While using the client library, I experienced issues where a successful serial connection would not be established.
To solve this, I added a short 4-second delay at the beginning of the mt_serial_init() function:

void mt_serial_init(int8_t rx_pin, int8_t tx_pin, uint32_t baud) {
    delay(4000);  // Needed to allow the LoRa device firmware to boot up
    // Serial initialization follows...
}

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}