Add a default callback to the PurpleCredentialProvider async methods.
Review Request #464 — Created Jan. 29, 2021 and discarded
If the caller didn't provide a callback the GTask and the result will remain in memory, so instead of just letting that leak we consume the tasks in the default handlers by calling the
_finishmethods and free the password if one was returned.
Compiled and verified everything was okay when removing an account (the only easy way to trigger one of the calls that don't have a callback).
Also, I couldn't add unit tests because since there's no callback, and I can't modify the default, everything just times out and makes the tests take forever.