feat: configure idle timeout for bun runtime.

```
[Bun.serve]: request timed out after 10 seconds. Pass `idleTimeout` to configure.
```

we need to be able to configure idle timeout for bun runtime, because if there are any [fetch](https://developer.mozilla.org/en-US/docs/Web/API/Window/fetch) inside event handler sometimes need more than 10 seconds.

we can leverage built in `NITRO_SHUTDOWN_TIMEOUT ` environment variable to configure this.

references:
- https://bun.sh/docs/api/http#idletimeout
- https://nitro.build/deploy/runtimes/node#environment-variables

src/presets/bun/runtime/bun.ts

@@ -12,6 +12,7 @@ const ws = import.meta._websocket
 
 // @ts-expect-error
 const server = Bun.serve({
+  idleTimeout: process.env.NITRO_SHUTDOWN_TIMEOUT || 10,
   port: process.env.NITRO_PORT || process.env.PORT || 3000,
   websocket: import.meta._websocket ? ws!.websocket : (undefined as any),
   async fetch(req: Request, server: any) {