chore(js): Allow triggering release on external artifact static_viewer.html

Previously, when a new version of static_viewer.html is released on the master
of vtk-js we couldn't trigger a release because we do not commit this file but
rather grab it during release. This commit adds the hash of the latest version
in a file and a hash check that runs during release but does not trigger a
failure on a mismatch but rather a warning.
So now if static_viewer.html is updated on vtk-js:
- If we do not care `.fetch_externals.sh` will just emit a warning
- If we **do** care we update .static_viewer.sha256 and trigger a new release
  on trame-vtk even if no other `trame-vtk` change happened during last
  release.

Author: ChristosT

.fetch_externals.sh

@@ -5,4 +5,8 @@ set -e
 mkdir -p ./trame_vtk/modules/common/serve
 curl https://unpkg.com/[email protected] -Lo ./trame_vtk/modules/common/serve/trame-vtk.js
 # echo 19e7c20470b952cc7b38f1434180b5ec ./trame_vtk/modules/common/serve/trame-vtk.js | md5sum -c --status && echo OK
-curl https://kitware.github.io/vtk-js/examples/OfflineLocalView/OfflineLocalView.html -Lo ./trame_vtk/tools/static_viewer.html
+curl https://kitware.github.io/vtk-js/examples/OfflineLocalView/OfflineLocalView.html -Lo ./trame_vtk/tools/static_viewer.html
+
+if ! sha256sum --check .static_viewer.sha256 ; then
+  echo "Hash for static_viewer.html differs, please update .static_viewer.sha256"
+fi

.static_viewer.sha256

@@ -0,0 +1 @@
+b757777855acdc69644186c0678fb8f15cd28614fc96454e567ee81ae57ff3be  ./trame_vtk/tools/static_viewer.html