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/server_message.schema.json

@@ -110,6 +110,18 @@
       ],
       "additionalProperties": false
     },
+    {
+      "type": "object",
+      "title": "CameraSnapshotRequest",
+      "description": "Read one JPEG frame from a printer's camera and send it back as a CameraSnapshot. For the printers whose camera is not RTSP (Bambu P1/A1, telemetry camera_snapshot true). Unlike WebrtcOffer this DOES carry the picture through the cloud, which is acceptable only because the frames are small, low-rate and requested one at a time.",
+      "properties": {
+        "type": { "const": "camera_snapshot_request" },
+        "request_id": { "type": "string" },
+        "printer_id": { "type": "string", "description": "The gateway's own printer id." }
+      },
+      "required": ["type", "request_id", "printer_id"],
+      "additionalProperties": false
+    },
     {
       "type": "object",
       "title": "ProvisionPrinter",