Marker interface. Implementations are used to exchange credentials between credential
verificator and credential retriever.
The actual exchange process might be complicated and require several interactions. E.g.
challenge based verificator might first give a challenge to the retrieval and then retrieval
return the challenge response. Typical implementation will define a method setting a callback.
The method will be implemented by verificator, and used by retriever.
Example credentials: SAML (IdP name/address), OpenID (IdP name/address), Username and password,
XX challenge, SMS