feat(mobile): use cached asset info if unchanged instead of downloading all assets (#1017)

* feat(mobile): use cached asset info if unchanged instead of downloading all assets

This adds an HTTP ETag to the getAllAssets endpoint and client-side support in the app.
If locally cache content is identical to the content on the server, the potentially large list of all assets does not need to be downloaded.

* use ts import instead of require

mobile/lib/utils/tuple.dart

@@ -0,0 +1,8 @@
+/// An immutable pair or 2-tuple
+/// TODO replace with Record once Dart 2.19 is available
+class Pair<T1, T2> {
+  final T1 first;
+  final T2 second;
+
+  const Pair(this.first, this.second);
+}