UART tx ringbuffer free space (IDFGH-642)

[sh83]

Problem Description

I am writing ardupilot hal for the esp32 series mcu (https://github.com/sh83/ardupilot/tree/esp32/libraries/AP_HAL_ESP32)
During uart driver development I was found what it is impossible to estimate amount of the avaliable tx ringbuffer space. This method is required in the ardupilot hal api (https://github.com/ArduPilot/ardupilot/blob/910de89b4e8b9b223af972fc682951afd998a4f5/libraries/AP_HAL/utility/BetterStream.h#L48), so for now I have to use internal buffers and move data back and forth in the dedicated thread.

Expected Behavior

There is function like esp_err_t uart_get_tx_free_space_len(uart_port_t uart_num, size_t* size);

Actual Behavior

No way to estimate free size of the tx ringbuffer.