[feature] Support multiple payees + payment types (subscription / one-time) in checkout

[brianfeister]

Overview

We want to expand our checkout flexibility to handle for multiple payment types and payees. Currently we only support one-time + single payee checkout.

The problem with this is that our business partner / customer venues in our network can't benefit from combined savings that sells "venue subscriptions" (e.g. "$10/mo and you get one free drink + other special perks).

We want to add Zeba Bar Membership as a purchasable (see purchasable schema here) to the checkout on this event

Notice that Zeba Bar seen as the Venue listed on the event above. We need to be able to do something similar to what The Regular App already does here:

https://theregularapp.adalo.com/the-regular-app?target=23wwime0grfjous8bvbe9j8fh&params=%7B"22xf9079ctjazwtsq4ni5l63d.t_aa6z143vhd7l786wn9kt20eo0"%3A9%7D

For UI components / we use

• https://daisyui.com/components/
• https://alpinejs.dev/

Requirements

• Changes will need to be made in purchasables.go to support payeeId on a per-item basis in the Purchasables[] array. Currently, payeeId is a field on the parent, but we need the child PurchasableItems in the array to have their own and to be able to differentiate the payout split between multiple targets (in this case https://meetnear.me/event/0c4eaa67-771c-4351-895f-0fc132c813cd - District Karaoke League AND Zeba Bar)
• Changes will need to be made in the checkout UI in navbar.templ to accommodate sending the new checkout payload
• Changes will be needed in the purchases service in order to account for the functional changes
• Changes will be needed in the CreateCheckoutSession API handler to account for the "dual checkout (both single item + Stripe subscription). Also note the Stack overflow issue where creating both types of item in the same checkout is discussed: https://github.com/meetnearme/api/blob/develop/functions/gateway/handlers/data_handlers.go#L822-L824