add dora-webcam node

Captures frames from a V4L2 camera and publishes them as std/media/v1/Image:
an Arrow struct of width, height, encoding and data, so a receiver needs
nothing from the message metadata. For a compressed encoding the recorded
dimensions are the image's, not the length of the payload.

Based on opencv-video-capture from dora-hub, with three changes. The output is
the typed image struct rather than a flat UInt8 buffer plus metadata. Camera
selection by id reads /dev/v4l/by-id/ only; the upstream macOS system_profiler
and Windows PowerShell enumeration is not carried. CAPTURE_FRAMES bounds a run
so a test ends on its own, in place of upstream's ten-second CI cutoff -- a
timeout would have made the CI step exit non-zero.

CI=true keeps a failed capture from raising, publishing a labelled black frame
instead, which is what lets dataflow.yml run on a machine with no camera.

Two dataflows, run one at a time, as dora has no cross-dataflow wiring:
dataflow.yml is the camera alone and the CI gate; dataflow_view.yml wires it
into dora-image-view and is not run by CI, since it fetches that node over the
network and needs a tag to exist there.

dataflow_view.yml

@@ -0,0 +1,23 @@
+# The camera into the viewer: the end-to-end check of the pair.
+#
+# Not run by CI. It fetches dora-image-view over the network, and it needs a
+# tag to exist in that repository; CI validates and runs dataflow.yml, which
+# needs neither.
+strict_types: true
+
+nodes:
+  - id: camera
+    path: dora-webcam
+    build: pip install .
+    inputs:
+      tick: dora/timer/millis/33
+    outputs:
+      - image
+
+  - id: view
+    git: ssh://git@gitea.liria.cl:5001/liria/dora-image-view.git
+    tag: v0.1.0
+    build: pip install .
+    path: dora-image-view
+    inputs:
+      image: camera/image