init_pin_async
Description:
Initialize the user's pin on this slot that this session is opened on.
According to the PKCS#11 standards, the session must be logged in with the `CKU_SO` user type.
This call will return immediately and completes asynchronously.
Parameters:
| this | Initialize PIN for this session's slot. | 
| pin | the user's PIN, or  | 
| cancellable | Optional cancellation object, or  | 
| callback | Called when the operation completes. | 
| n_pin | the length of the PIN | 
| user_data | Data to pass to the callback. |