Add client functions to allow integrations within other CLIs.

This is a simplification of how the docker engine implements
this feature, but it will be ported there once this is merged.

Signed-off-by: David Calavera <david.calavera@gmail.com>

credentials/helper.go

@@ -1,14 +1,5 @@
 package credentials
 
-import "errors"
-
-// Credentials holds the information shared between docker and the credentials store.
-type Credentials struct {
-	ServerURL string
-	Username  string
-	Secret    string
-}
-
 // Helper is the interface a credentials store helper must implement.
 type Helper interface {
 	// Add appends credentials to the store.
@@ -19,7 +10,3 @@ type Helper interface {
 	// It returns username and secret as strings.
 	Get(serverURL string) (string, string, error)
 }
-
-// ErrCredentialsNotFound standarizes the not found error, so every helper returns
-// the same message and docker can handle it properly.
-var ErrCredentialsNotFound = errors.New("credentials not found in native keychain")