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>

docs/sidebarTutorials.js

@@ -88,6 +88,7 @@ const sidebars = {
         'extensions/deployment',
         'extensions/mcp',
         'extensions/security',
+        'extensions/storage',
         'extensions/tasks',
         'extensions/registry',
       ],