Add Optional Setting for "Force Only 1 Subscription at a Time"

For sure, it's really cool that a customer can purchase more than one subscription at a time in FluentCart (e.g., those selling plugin subscriptions). But for membership course operators (like me), I foresee some customer support nightmares in our future!

I'm doing my best to use the "simple variations" and "upgrade path" options for subscription products so that members can use the "Upgrade Plan" button on their account subscription (tab) page. And in a perfect world, this is all they would ever click to change to a different plan type or price. But we know this isn't going to be the case.

I'm a bit concerned some of my members, instead, will think they are upgrading (replacing) their current subscription correctly when they go back to my membership sales page and they click on a different membership (buy button) than they currently have, or they find a link to a discounted version of my membership (a completely different membership subscription product with different pricing) than they currently have (maybe in a promotional email)... which will take them to the checkout with a new subscription. Then a month (or even more) later, they check their credit card statements and they realize they have two memberships running at the same time — and they have been paying for both!!!

Now I have to explain, apologize, and figure out how to manually refund or manually add time to their subscription plan.

I think we might need a setting: "Only allow customers to have 1 subscription at a time." (I see there's a product setting to "limit purchases to 1 item per order" — which is great! But this is completely different and also needed)

Or, maybe a more correct way to implement this would be to have an optional "Only 1 at a Time" category checkbox on each of our subscription products, so that if a customer later tries to add another subscription product also marked "Only 1 at a Time," FluentCart will then know it needs to replace their current subscription with the new one in the checkout process. This alternate method would allow some of our global premium (membership type) subscriptions to be sold only one at a time, while also having other (maybe minor) subscription types be sold alongside at the same time (e.g., addon "VIP" Community spaces, or addon course lessons not included in our main membership pricing)

I would also propose that when a customer, currently already with a subscription, goes to checkout with another subscription, their new subscription is pro-rated (discounted) based on how much "paid" time they have left on their current subscription. (Like how the "Upgrade Plan" button works)