chore(checkout-v3): add feature flag (#97520)

Closes
https://linear.app/getsentry/issue/BIL-1200/create-feature-flag-for-new-checkoutsubscriptions-ui

Author: isabellaenriquez

src/sentry/features/temporary.py

@@ -553,14 +553,16 @@ def register_temporary_features(manager: FeatureManager):
     manager.add("organizations:github-multi-org", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=False)
     # Enable seeing upsell modal when clicking upgrade for multi-org
     manager.add("organizations:github-multi-org-upsell-modal", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
-    # Enables quick testing of disabling transaction name clustering for a project.
-    manager.add("projects:transaction-name-clustering-disabled", ProjectFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=False, default=False)
+    # Enables new billing-related UI (checkout, subscription page)
+    manager.add("organizations:checkout-v3", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
 
     # NOTE: Don't add features down here! Add them to their specific group and sort
     #       them alphabetically! The order features are registered is not important.
 
     # Project scoped features #
     ###########################
+    # Enables quick testing of disabling transaction name clustering for a project.
+    manager.add("projects:transaction-name-clustering-disabled", ProjectFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=False, default=False)
     # Adds additional filters and a new section to issue alert rules.
     manager.add("projects:alert-filters", ProjectFeature, FeatureHandlerStrategy.INTERNAL, default=True)
     manager.add("projects:discard-transaction", ProjectFeature, FeatureHandlerStrategy.INTERNAL, api_expose=False)