R1:5a8a56f414fc

Prepare the new AuthPassword infrastructure for storing account passwords

Summary:
Ref T13043. In D18898 I moved VCS passwords to the new shared infrastructure.

Before account passwords can move, we need to make two changes:

- For legacy reasons, VCS passwords and Account passwords have different "digest" algorithms. Both are more complicated than they should be, but we can't easily fix it without breaking existing passwords. Add a `PasswordHashInterface` so that objects which can have passwords…