Allow for anonymous customer_id in checkout

[peteristhegreat]

Feature request

• Extension name: firestore-stripe-payments

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

I am making a checkout system similar to a POS device. If the user wants a receipt, then they put in their email. If they don't care, or don't want it, then they don't put in their email.

Describe the solution you'd like

Stripe supports guest users, if the customer_id is null. Can this get supported in this library?

Maybe the location for guest checkouts could be all under a single customer id "guest" or "anonymous", so the checkout sessions and payment intents are still stored how they are currently.

Describe alternatives you've considered

#94

This looks like a similar-ish idea, but for some other niche case.

I already am using Do Not Sync option. I want it to go one step further and not require the customer_id to be set at all.

Another alternative I've looked at is creating a hashed email based on pieces of information from their form... But this is still just a bogus email to create a firebase auth login to let everything work.

Additional context

Thanks for the great library and for the maintenance on it.