config: esp32: configurable UART FIFO thresholds

The existing configuration of the ESP32 UART FIFO thresholds
was fixed, leading to inefficiencies in handling
Modbus RTU packages exceeding this size.
This commit introduces two new Kconfig options,
allowing users to adjust the esp32 fifo thresholds as needed.
fixes #74311

Signed-off-by: Omer Gozderesi <omer.gozderesi@enda.com>

config: esp32:  configurable UART FIFO thresholds

The existing configuration of the ESP32 UART FIFO thresholds
was fixed, leading to inefficiencies in handling
Modbus RTU packages exceeding this size.
This commit introduces two new Kconfig options,
allowing users to adjust the esp32 fifo thresholds as needed.
fixes #74311
Signed-off-by: Omer Gozderesi <omer.gozderesi@enda.com>