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>

.github/workflows/preview_build.yml

@@ -17,6 +17,20 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps:
+
+      - name: Comment PR
+        uses: thollander/actions-comment-pull-request@v2
+        env:
+          URL_SLUG: ${{ github.event.number }}-${{ github.head_ref }}
+        with:
+          message: |
+            Preview is being built...
+            
+            Preview will be available at https://egui-pr-preview.github.io/pr/${{ env.URL_SLUG }}
+            
+            View snapshot changes at [kitdiff](https://rerun-io.github.io/kitdiff/?url=${{ github.event.pull_request.html_url }})
+          comment_tag: 'egui-preview'
+
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
         with: