private static String encodeClientCredentials(String clientId, String clientSecret) {
        return Base64.getEncoder()
                     .encodeToString(
                             (clientId + CREDENTIALS_SEPARATOR + clientSecret)
                                     .getBytes(StandardCharsets.UTF_8));
    }
Converts this {@link ClientAuthorization} to a {@link ClientAuthentication}.
oauth2/src/main/java/com/linecorp/armeria/common/auth/oauth2/ClientAuthorization.java