[amp-story-shopping] Allow products to have a pricing interval for subscriptions

### Description

Currently the amp-story-shopping component only supports products with a fixed (or discounted, when #37949 is ready) price. However, a growing number of products and services are being sold as a subscription.

I would suggest adding an optional `productInterval` enum that can be one of `month`, `year`, `week`, `day` or `hour` and an optional `productIntervalCount` int that defaults to 1 if the `productInterval` is set.

This would allow price modelling such as `50$ every 3 months`, `200$ per hour`, `9.99$ per month`, `29$ per day`, etc.

### Alternatives Considered

None, this would be a new feature