feat(uart): support UART Tx Rx pin swap function

[fronders]

This enables UART Tx and Rx pin swap function on STM32 families that support it (F0, L0, L4, G0, G4, H7 etc.)

In order to enable pin swap, user just needs to swap the pins either when declaring HardwareSerial object:

//                      RX    TX
HardwareSerial Serial1(PA10, PA9); // uses normal pins
//                      RX    TX
HardwareSerial Serial1(PA9, PA10); // uses swapped pins

or by using setTx() and setRx() methods:

Serial1.setRx(PA9);  // normally it is a Tx pin
Serial1.setTx(PA10); // normally it is a Rx pin
Serial1.begin(9600);

If the chip does not support the swap, or the pins are incorrect, the library behaves the same way as before (throws an error for invalid pins).

The cool thing is that it even supports half-duplex mode on the normally Rx pin - the Rx pin is used for transmission and reception then. User can either use Rx == Tx trick as before or just provide a single pin definition

Fixes: #2538
See also: #1418

[fronders]

Hi @fpistm, I'm finishing testing at the moment with different STM32 chips and will report back as soon as I'm done and things are ready to be merged :)