gptimer_get_resolution() function

- ESP_OK: Get GPTimer resolution successfully - ESP_ERR_INVALID_ARG: Get GPTimer resolution failed because of invalid argument - ESP_FAIL: Get GPTimer resolution failed because of other error

usually the timer resolution is same as what you configured in the `gptimer_config_t::resolution_hz`, but some unstable clock source (e.g. RC_FAST) will do a calibration, the real resolution can be different from the configured one.