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>

secretservice/secretservice_linux_test.go

@@ -43,7 +43,7 @@ func TestMissingCredentials(t *testing.T) {
 
 	helper := Secretservice{}
 	_, _, err := helper.Get("https://adsfasdf.wrewerwer.com/asdfsdddd")
-	if err != credentials.ErrCredentialsNotFound {
-		t.Fatalf("exptected ErrCredentialsNotFound, got %v", err)
+	if !credentials.IsErrCredentialsNotFound(err) {
+		t.Fatalf("expected ErrCredentialsNotFound, got %v", err)
 	}
 }