feat(extensions): add Tier 1 and Tier 2 storage APIs for extensions

Implement managed storage APIs for extensions with automatic namespace
isolation. Storage is automatically bound to extensions before module
execution, ensuring data privacy between extensions.

- Tier 1 (localState/sessionState): Browser-based storage with user isolation
- Tier 2 (ephemeralState): Server-side cache with TTL support
- Update webpack externals to support subpath imports like @apache-superset/core/storage
- Add storage documentation and update architecture docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

superset-extensions-cli/src/superset_extensions_cli/templates/frontend/webpack.config.js.j2

@@ -26,8 +26,13 @@ module.exports = (env, argv) => {
       extensions: [".ts", ".tsx", ".js", ".jsx"],
     },
     externalsType: "window",
-    externals: {
-      "@apache-superset/core": "superset",
+    externals: ({ request }, callback) => {
+      // Map @apache-superset/core and subpaths to window.superset
+      if (request?.startsWith("@apache-superset/core")) {
+        const parts = request.replace("@apache-superset/core", "superset").split("/");
+        return callback(null, parts);
+      }
+      callback();
     },
     module: {
       rules: [