SEC-31363: GA Proofpoint On-Demand Email Security integration (DataDog#23943)

Flips display_on_public_website to true in proofpoint_on_demand/manifest.json
and adds proofpoint_on_demand/assets/dataflows.yaml with granular: true for
Cloud SIEM.

Author: fabrequentin

proofpoint_on_demand/assets/dataflows.yaml

@@ -0,0 +1,6 @@
+provides:
+  - id: proofpoint-on-demand-logs
+    always_on: true
+    granular: true
+    data_type: logs
+    direction: inbound

proofpoint_on_demand/manifest.json

@@ -3,7 +3,7 @@
   "app_uuid": "1cef2070-562b-4d59-b035-ea26d6a2cd75",
   "app_id": "proofpoint-on-demand",
   "owner": "saas-integrations",
-  "display_on_public_website": false,
+  "display_on_public_website": true,
   "tile": {
     "overview": "README.md#Overview",
     "configuration": "README.md#Setup",