retrieve_secret
Description:
Retrieve the secret value of this object.
Each retrievable object has a single secret which might be a password or some other secret binary value.
This function returns immediately and completes asynchronously.
Parameters:
| this |
a retrievable object |
| cancellable |
optional cancellation object |
| callback |
called when the operation completes |
| user_data |
data to pass to the callback |