Add Version to `encryption.Key.ID()` method

Created by: arussellsaw

Right now the ID() method on encryption.Key returns a single string identifying the whole key, i think it's worth splitting this out into key name & key version before we start using encryption in anger, as i imagine there will be times that we want to identify keys by name without version, and we won't have a reliable heuristic to determine that from an ID string.

type Key interface {
  Encrypter
  Decrypter
  Key(ctx context.Context) (KeyVersion, error)
}

type KeyVersion struct {
  Name string
  Version string
}