Introduce CredentialStore which works like FileSystemSecurityRealm

It would be useful and convenient to have a credential store implementation using the same techniques – and ideally the same storage format and backend – as FileSystemSecurityRealm. Creating a basic abstraction of this format might be a reasonable first step, or it might make sense to implement either the realm in terms of the credential store, or the credential store in terms of the realm.

The credential store would probably not make any use of the attributes.