[Wishlist] Allow Custom Stripe Billing Portal Config Instead of Auto-Generated One

The Problem
Memberstack automatically creates a Stripe Billing Portal configuration for customers, but subscription updates are disabled by default. This prevents members from upgrading or downgrading their plans. The user also cannot control which specific prices/plans appear in the Memberstack-generated Billing Portal. As a result, customers are being sent to a portal config with "subscription_update": {"enabled": false} and incorrect plan options.

Why It's Important
Customers need a way to manage their subscriptions without contacting support. Not being able to upgrade or downgrade leads to a poor customer experience, lost revenue opportunities, and extra manual work for the site owner. The user already has a fully configured default Stripe Billing Portal that supports subscription updates, but Memberstack does not currently allow using or mirroring that configuration.

What Have You Tried?
The user attempted to edit the Memberstack-generated Billing Portal in Stripe, but Memberstack overwrites it automatically. They also tried mapping the correct prices manually, but the portal config assigned by Memberstack doesn’t allow subscription updates and cannot be replaced. There is no available workaround through the dashboard or API.

Suggested Solutions
Allow Memberstack users to:
• Enable or disable subscription updates in the Billing Portal config created by Memberstack.
• Select which plans/prices appear as upgrade/downgrade options.
• Optionally choose an existing Stripe Billing Portal config (such as their default) to use instead of the autogenerated one.

This would give users control over the subscription experience and prevent misalignment with their Stripe settings.