Carry camera stream information on the edge protocol

Two optional fields so a user can be pointed at a printer's camera.

`hello.stream_base_url` is where the gateway serves restreams - the base of
the go2rtc instance it already supervises. Reported by the gateway rather
than configured in the cloud, because only the gateway knows the address it
is actually reachable at.

That address is a LAN one, and the schema says so where someone will read it
rather than leaving it to be discovered in a support ticket. A browser on the
same network as the gateway can play the stream; one outside it cannot, and
the relay that would fix it does not exist. For a print farm whose operator
is in the workshop this is the normal case and it works; for anyone else it
is a limit, and anything surfacing the field has to cope with it being
unreachable.

`printer_telemetry.camera_stream` is the stream's name, present only when
there is one. Absent is the common answer today and the reason is worth
recording: a Bambu P1/A1 publishes no RTSP URL at all - a real P1P's ipcam
block carries `ipcam_dev` and `ipcam_record` and nothing to connect to -
while an H2C reports `rtsps://<ip>:322/streaming/live/1`. So RTSP restreaming
covers the X1/H2 class and nothing else; the rest need Bambu's proprietary
port-6000 JPEG stream, which is a client no gateway has yet.

Both optional and additive, so existing gateways are unaffected.

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

@@ -20,6 +20,11 @@
         },
         "token": {
           "type": "string"
+        },
+        "stream_base_url": {
+          "type": "string",
+          "format": "uri",
+          "description": "Where this gateway serves camera restreams, e.g. http://gateway.local:1984 - the base of a go2rtc instance the gateway supervises. Optional: a gateway with no cameras configured, or with go2rtc not installed, omits it. Reported by the gateway rather than configured in the cloud because only the gateway knows the address it is actually reachable at. NOTE the consequence, which is a real limit and not a bug: this is a LAN address. A browser on the same network as the gateway can play the stream directly; one outside it cannot, and making that work needs a relay that does not exist yet. Anything surfacing this to a user has to cope with it being unreachable."
         }
       },
       "required": [