Feature request

• Extension name: Firestore Stripe Payments Web SDK

Is your feature request related to a problem? Please describe.

Customers cannot adjust quantities during checkout because there is no way to enable adjustable quantity when calling the createCheckoutSession function.

Describe the solution you'd like

Currently createCheckoutSession accepts line_items, which has properties of price, quantity, and description.

I would also like to add the adjustable_quantity property to line_items as described here so customers can adjust line item quantity during checkout.

For example:

line_items: [
  {
    price: priceId,
    description: 'Buy this thing',
    adjustable_quantity: {
      enabled: true,
      minimum: 1,
      maximum: 10
    }
    quantity: 1,
  },
],

Describe alternatives you've considered

I could create my own function, but that would be more fragile and also defeats the purpose of using the Firestore Stripe Payments Web SDK which is supposed to abstract these things away.