gptimer_get_raw_count() function

- ESP_OK: Get GPTimer raw count value successfully - ESP_ERR_INVALID_ARG: Get GPTimer raw count value failed because of invalid argument - ESP_FAIL: Get GPTimer raw count value failed because of other error

This function will trigger a software capture event and then return the captured count value. With the raw count value and the resolution returned from `gptimer_get_resolution`, you can convert the count value into seconds. This function is allowed to run within ISR context If `CONFIG_GPTIMER_CTRL_FUNC_IN_IRAM` is enabled, this function will be placed in the IRAM by linker, makes it possible to execute even when the Flash Cache is disabled.