Feat/stripe subscription proxy

[martijnvdbrug]

Description

• Added a proxy function for retrieving created subscriptions in the current channel.
• The function proxies incoming arguments to the Stripe client. See available parameters here: https://stripe.com/docs/api/subscriptions/list
• Example usage:

import { StripeSubscriptionService } from `@pinelab/vendure-plugin-stripe-subscriptions`;

// There are multiple ways to onbtain an injector instance in your current context. See https://docs.vendure.io/typescript-api/common/injector/
injector.get(StripeSubscriptionService).getAllSubscriptions(ctx);

// Or, get a single subscription
injector.get(StripeSubscriptionService).getSubscription(ctx, 'sub_1No3b9225jfF0VNwOedpxA2l');

• StripeClient can now be obtained with just a RequestContext, no order ID needed anymore.

Checklist

📌 Always:

• I have set a clear title
• My PR is small and contains only a single feature. (Split into multiple PR's if needed)
• I have reviewed my own PR, fixed all typo's and removed unused/commented code

⚡ Most of the time:

• I have added or updated test cases for important functionality
• I have updated the README if needed

📦 For publishable packages:

• Have you correctly increased the version number in package.json to the next patch/minor/major?
• Have you added your changes to the changelog? See this example