Windows custom ui extension workaround

[HuakunShen]

From

• BUG: Extension Template works in browser but fails within KunKun App - "File not found" Error Displayed kunkun-nuxt#79

Tauri's implementation for custom protocol on Windows has a problem that denies requests from within iframe.

• [bug] Inconsistent register_uri_scheme_protocol behaviour on Windows. Connection refused for requests from iframe. tauri-apps/tauri#11505
• `register_uri_scheme_protocol` behaves differently on different platforms! How to get a real custom uri scheme on Windows? tauri-apps/tauri#10868
  It doesn't seem like there will be a fix from Tauri, so I have to implement a workaround for windows.

Candidate Solutions
Since custom protocol won't work, the only option is to serve files from http file server.
But I don't really want to revert to the old design where each extension need base url configuration (so ugly and hard to maintain).

Disable multi-window support for extensions, i.e. only one extension at a time; extension only loads in main window
Still support multi-window, then spawn a new file server per custom ui extension, and kill it after window is closed
Make use of the window extension hash map stored in app state. Maybe simply also store an axum server handle in the hash map
Existing code will remove a window label from the map when extension quits, just kill the server with server handle here
For now I'd choose option 2 (multi server)

There could be multiple servers simultaneously, which takes more ports, and more RAM.
However, since all servers on in a single process, but different thread, and the workload isn't heavy (just serve static files), I guess the extra RAM usage won't be too much (experiment required)