We are removing dependencies on docker types where possible in the core
libraries. credentialprovider is generic to Docker and uses a public API
(the config file format) that must remain stable. Create an equivalent type
and use a type cast (which would error if we ever change the type) in the
dockershim. We already perform a transformation like this for CRI and so
we aren't changing much.
Credential provider is useful without the v1 API, move the only
dependency out so that we can more easily move credential provider to a
utility library in the future (other callers besides Kubelet may need to
load pull secrets like Docker).
