Auto-initialize hardware flow control for SerialN objects

[mbedNoobNinja]

This PR adds automatic hardware flow control initialization for UART SerialN objects.

Previously, users had to manually construct a UART object to enable RTS/CTS:

UART BleSerial(
    UART2_TX_PIN,
    UART2_RX_PIN,
    UART2_RTS_PIN,
    UART2_CTS_PIN
);

void setup() {
    BleSerial.begin(115200);
}

void loop()
{
    ...
}

This is inconvenient because the board variant already defines the RTS and CTS pins.

With this change, calling

Serial2.begin(...);

automatically enables hardware flow control whenever both RTS and CTS pins are defined for the selected UART in pins_arduino.h. (I specifically, needed it for this board VK-RA4M2-NANO, on Serial2)

This keeps the API simple while preserving the existing behavior for boards that do not define hardware flow control pins.

[CLAassistant]

All committers have signed the CLA.