Add camera snapshot messages and flag for P1/A1 cameras

camera_snapshot_request (cloud to gateway) and camera_snapshot (gateway
to cloud) carry one JPEG frame at a time; telemetry gains camera_snapshot
and the app view gains cameraSnapshot.

schemas/continuum/v1/edge/client_message.schema.json

@@ -183,6 +183,18 @@
         "message"
       ]
     },
+    {
+      "type": "object",
+      "title": "CameraSnapshot",
+      "description": "The answer to a CameraSnapshotRequest: one JPEG frame (base64), or the reason there is none.",
+      "properties": {
+        "type": { "const": "camera_snapshot" },
+        "request_id": { "type": "string" },
+        "jpeg_base64": { "type": "string" },
+        "error": { "type": "string", "description": "Set instead of jpeg_base64 when no frame could be read." }
+      },
+      "required": ["type", "request_id"]
+    },
     {
       "type": "object",
       "title": "DiscoveredPrinters",