Provide subscription.js with selfServiceToken

Problem/Motivation

As a helper for frontend actions based on subscriptionJS: https://developer.billwerk.io/docs/subscriptionJS/overview
it might be helpful to provide it from the module including the helpers to get the selfServiceToken for the user.

See https://developer.billwerk.io/docs/subscriptionJS/setup

Might be provided as Drupal external library definition?

The selfServiceToken retrieval is already available via

  public function getSelfserviceTokenArray(string $contractId): array {
    return $this->get(self::$contractResource . $contractId . "/selfServiceToken");
  }

in BillwerkApi.php

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes