Adjust the getCredential signature to List<String> credentialNames, Map<Credential,Set<String>

The Map being the accepted credential types along with acceptable algorithms - this method will only be able to return Credential.

An overloaded method will be added to obtain by name and type only - we could even have an overloaded method that just takes the name and returns Credential. These overloaded methods can all have default implementations leaving the realm only required to implement the complete form.