async createSubscription(customer, price) {
        await this._rateLimitBucket.throttle();
        const subscription = await this._stripe.subscriptions.create({
            customer,
            items: [{price}]
        });
        return subscription;
    }
Create a new Stripe Subscription for a Customer by ID and Price ID.

@param {string} customer - The ID of the Customer to create the subscription for
@param {string} price - The ID of the new Price

@returns {Promise<ISubscription>}