add back in volumeDecimal - #35

for backwards compatibility while we upgrade our Stripe API.

do we really need this? we can change the fieldname at the same time as we pump version on server side, can't we?

@andreysundukov - so we're unsure of how we actually get Stripe API to upgrade its version (see https://getstep.slack.com/archives/C063CMD01AM/p1725453483821499). Supposedly to upgrade Stripe API version we need to manually do that on Stripe's dashboard (although we appear to never have done this and most of our traffic is on a relatively recent version). In the interim between when we upgrade the SDK version in server and us upgrading the API version, I was thinking there's a possibility that Stripe may still send us volumeDecimal instead of quantityDecimal which we would miss persisting, so as a stopgap we can have both and server can try to persist whichever it receives. Although tbh, it doesn't seem like we are ever persisting fuel at all so this may not end up mattering, but figured it is better to be extra safe.