sync-service: switch to SecretRetrievable API
secret_password_search_finish() returns a list of SecretRetrievable objects. We incorrectly assume it returns a list of SecretItems. SecretItem seems to be the only public implementation of SecretRetrievable, but clearly it's not the only implementation.
We get to the same end result -- a SecretValue object -- either way, but SecretRetrievable has an async interface, so the code is a bit more complicated now. It's surely for the better.
Hopefully/probably fixes #1361 (closed)