Get a Backend instance.
If such a backend already exists, then the same backend is returned.
If flags contains any flags of which parts of the secret backend to ensure are initialized, then those will be initialized
before completing.
This method will return immediately and complete asynchronously.
| flags |
flags for which service functionality to ensure is initialized |
| cancellable |
optional cancellation object |
| callback |
called when the operation completes |
| user_data |
data to be passed to the callback |