Add workflow to accept snapshots via kitdiff (#7577)

This adds a new workflow `update_kittest_snapshots.yml` that can be
triggered through the [kitdiff](https://github.com/rerun-io/kitdiff) ui
when viewing a ci artefact.
Also adds a link to kitdiff to view the pr changes to each commit (via
the preview build comment)

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

scripts/accept_snapshots.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+# This script moves all {name}.new.png files to {name}.png.
+# Its main use is in the update_kittest_snapshots CI job, but you can also use it locally.
+
+set -eu
+
+# rename the .new.png files to .png
+find . -type d -path "*/tests/snapshots*" | while read dir; do
+    find "$dir" -type f -name "*.new.png" | while read file; do
+        mv -f "$file" "${file%.new.png}.png"
+    done
+done
+
+echo "Done!"