driver: add rs485 half duplex interface support to uart driver (remove ignore tag...

driver: add rs485 half duplex interface support to uart driver (remove ignore tag to check ut case)

An existing UART driver does not support RS485 half duplex mode.
This task adds this functionality to ESP_IDF UART driver.
driver/uart.c/h: updated to add support of RS485 half duplex mode
examples/peripherals/uart_echo_rs485/main/rs485_example.c: added test example
components/driver/test/test_uart.c: added test of RS485 half duplex mode
docs/en/api-reference/peripherals/uart.rst: updated documentation
test_uart.c: suppress GCC warnings about discarded const qualifiers
uart.rst: remove sphinx warning - "Duplicate explicit target name"
simple change in uart.h file
update (test_uart.c) after rebase from master
update uart.rst, uart.c, rs485_example.c
Update example description in file Readme.md
update uart.c/h, uart.rst, test_uart.c according to review results
update uart.h (uart_set_rx_timeout() description
test_uart.c remove ignore tag

The tests are completed using RS485 adapters hardware connected to two ESP32 WROVER KITs.

TW#13812
Closes https://github.com/espressif/esp-idf/pull/667
Closes https://github.com/espressif/esp-idf/pull/1006