Server Function Timeout

## Feature Request

```rs
#[server(timeout_secs=1.5)] // FeatureRequest 1: timeout attribute
async fn remote_call() -> Result {
// slow operation
}

match remote_call().await {
Err(ServerFnError::Timeout) => ...
...
}

// // FeatureRequest 2: with_timeout combinator
match remote_call().with_timeout(Duration::..).await {
...
}

```

Setting a timeout from a caller side with gloo_timers has disadvantages:
- The call site becomes very verbose
- the internal POST request for RPC cannot be explicitly cancelled on timeout

## Implement Suggestion

```rs
use gloo_timers::future::TimeoutFuture;

match select(rpc_call, TimeoutFuture::new(ms)).await {
Either::Left((response, timer)) => {
drop(timer);
return Ok(response);
}
Either::Right((rpc_call, _)) => {
rpc_call.cancel();
return Err(ServerFnError::Timeout)
}
}
```

Start by locating the server-function RPC handling and ServerFnError definitions, then assess the proposed timeout attribute and with_timeout combinator against the cancellation approach shown. Done should be defined by deciding which feature scope to implement and how timed-out calls report ServerFnError::Timeout.

Written by the indexing model from the issue text.