Revenue Cat and OneSignal plugins

[clsource]

These services are popular within indie developers to gather payments and push notifications.
It would be great to have plugins that integrate with these services and quickly configure them using mob

• https://www.revenuecat.com/
• https://onesignal.com/

[GenericJam]

Triage: keeping open as a plugin-roadmap item. Two halves:

• In-app purchase — mob_in_app_purchase is scaffolded (StoreKit 2 / Play Billing surface). RevenueCat would sit on top of that as a higher-level plugin.
• OneSignal (push) — mob_push already exists for APNs/FCM; a OneSignal plugin would be an alternative provider.
  No work scheduled yet; leaving open to track the direction.