Fixes UART pin setting + adds CTS/RTS HW Flow Control

[SuGlider]

Summary

This PR has two components:
1- Fixes Serial.begin() in order to keep RX/TX pins the same, as previouly set, when those parameter are not used.
2 - Adds Hardware Flow Control and adds RTS/CTS pins setting

Impact

Serial will keep RX/TX pins unmodified in case of several begin() calls:

void setup() {
    Serial1.begin(115200, SERIAL_8N1, 18, 19);
    Serial1.println("HELLO WORLD!");
  
    Serial1.begin(115200);
    Serial1.begin(115200);
    Serial1.println("HELLO WORLD!  2nd Time!");
}

Modified and New functions - from HardwareSerial class:

    // Negative Pin Number will keep it unmodified, thus this function can set individual pins
    // SetPins shall be called after Serial begin()
    void setPins(int8_t rxPin, int8_t txPin, int8_t ctsPin = -1, int8_t rtsPin = -1);
    // Enables or disables Hardware Flow Control using RTS and/or CTS pins (must use setAllPins() before)
    void setHwFlowCtrlMode(uint8_t mode = HW_FLOWCTRL_CTS_RTS, uint8_t threshold = 64); 

Note: setPins() accepts negative pin number that will keep it unmodified.

Related links

This replaces PR #6176
Fixes #6185

[P-R-O-C-H-Y]

LGTM

[mrengineer7777]

Possible regression from #6176 . _rxPin(-1), _txPin(-1) were removed. How are the previous pin states tracked?

[SuGlider]

@mrengineer7777
No need to track.
It is all done by IDF instead.

Please test the PR to check that it works as intended and let me know.

The "pin tracking" will get lost only when Serial.end() is called, but it sounds to be the correct expected behavior.

[mrengineer7777]

Please test the PR to check that it works as intended and let me know.

@SuGlider Apologies, as this isn't my issue and I'm not an Espressif employee I won't be able to test this.

The "pin tracking" will get lost only when Serial.end() is called, but it sounds to be the correct expected behavior.

Seems reasonable.

[me-no-dev]

always add default: for switch statements. ESP-IDF will complain

[SuGlider]

done.

[me-no-dev]

If this enum will be used just for the switch statement, let's use int instead

[SuGlider]

It is solved now by including "driver/uart.h" that has those defines.