feat(sentry-apps): Register sentry-apps-custom-webhook-headers feature flag

Gate delivery of custom webhook headers (and the frontend settings
field) behind a Flagpole organization feature flag so rollout can be
controlled per-org.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: billyvg

src/sentry/features/temporary.py

@@ -308,6 +308,8 @@ def register_temporary_features(manager: FeatureManager) -> None:
     manager.add("organizations:seer-gitlab-support", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
     # Run Seer agents inside the sandbox execution environment
     manager.add("organizations:seer-use-agent-sandbox", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=False)
+    # Enable delivery of custom webhook headers configured on a SentryApp
+    manager.add("organizations:sentry-apps-custom-webhook-headers", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
     # Disable select orgs from ingesting mobile replay events.
     # Enable double-read from EAP for session health data validation
     manager.add("organizations:session-health-eap", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=False)