Implement client.List, change list API

[]string, []string -> map[string]string because the other APIs assume a
1:1 correspondence

Signed-off-by: Jake Sanders <jsand@google.com>

client/client.go

@@ -68,14 +68,20 @@ func Erase(program ProgramFunc, serverURL string) error {
 	return nil
 }
 
-// List executes a program to remove the server credentials from the native store.
-func List(program ProgramFunc) error {
+// List executes a program to list server credentials in the native store.
+func List(program ProgramFunc) (map[string]string, error) {
 	cmd := program("list")
-	cmd.Input(strings.NewReader("garbage"))
+	cmd.Input(strings.NewReader("unused"))
 	out, err := cmd.Output()
 	if err != nil {
 		t := strings.TrimSpace(string(out))
-		return fmt.Errorf("error listing credentials - err: %v, out: `%s`", err, t)
+		return nil, fmt.Errorf("error listing credentials - err: %v, out: `%s`", err, t)
 	}
-	return nil
+
+	var resp map[string]string
+	if err = json.NewDecoder(bytes.NewReader(out)).Decode(&resp); err != nil {
+		return nil, err
+	}
+
+	return resp, nil
 }