pcnt_unit_register_event_callbacks() function

- ESP_OK: Set event callbacks successfully - ESP_ERR_INVALID_ARG: Set event callbacks failed because of invalid argument - ESP_ERR_INVALID_STATE: Set event callbacks failed because the unit is not in init state - ESP_FAIL: Set event callbacks failed because of other error

User registered callbacks are expected to be runnable within ISR context The first call to this function needs to be before the call to `pcnt_unit_enable` User can deregister a previously registered callback by calling this function and setting the callback member in the `cbs` structure to NULL.