Fix https / http asset loader on WASM (#21283)

# Objective

Our `web_asset` example does not build on Wasm. Ironic.

As reported in #21280, there are a few problems:

1. Imports are broken: one hard error and a few warnings.
2. `HttpWasmAssetReader` needs a `+ use<'>` to conform to the Rust 2024
edition (unreported, but discovered)
3. We're returning a reference to a temporary value, which is ultimately
caused by 2

## Solution

1. Move the imports into the correct feature-gated block.
2. Slap some `+ use<'>` on the function definition like the compiler
told me to. This fixes the "returning a reference to a temporary value"
problem, as we're no longer implicitly capturing.

Fixes #21280.

Special thanks to @kristoff3r for pointing out my mistake!

## Testing

I've tested this PR using `bevy run --features https --example web_asset
web` via the bevy_cli. Because of changes to `getrandom`, this was
non-trivial to build for web manually.

Author: alice-i-cecile

crates/bevy_asset/src/io/wasm.rs

@@ -53,7 +53,10 @@ fn js_value_to_err(context: &str) -> impl FnOnce(JsValue) -> std::io::Error + '_
 
 impl HttpWasmAssetReader {
     // Also used by [`WebAssetReader`](crate::web::WebAssetReader)
-    pub(crate) async fn fetch_bytes(&self, path: PathBuf) -> Result<impl Reader, AssetReaderError> {
+    pub(crate) async fn fetch_bytes(
+        &self,
+        path: PathBuf,
+    ) -> Result<impl Reader + use<>, AssetReaderError> {
         // The JS global scope includes a self-reference via a specializing name, which can be used to determine the type of global context available.
         let global: Global = js_sys::global().unchecked_into();
         let promise = if !global.window().is_undefined() {

crates/bevy_asset/src/io/web.rs

@@ -1,10 +1,9 @@
 use crate::io::{AssetReader, AssetReaderError, Reader};
 use crate::io::{AssetSource, PathStream};
 use crate::{AssetApp, AssetPlugin};
-use alloc::{borrow::ToOwned, boxed::Box};
+use alloc::boxed::Box;
 use bevy_app::{App, Plugin};
 use bevy_tasks::ConditionalSendFuture;
-use blocking::unblock;
 use std::path::{Path, PathBuf};
 use tracing::warn;
 
@@ -124,8 +123,9 @@ async fn get<'a>(path: PathBuf) -> Result<Box<dyn Reader>, AssetReaderError> {
 #[cfg(not(target_arch = "wasm32"))]
 async fn get(path: PathBuf) -> Result<Box<dyn Reader>, AssetReaderError> {
     use crate::io::VecReader;
-    use alloc::{boxed::Box, vec::Vec};
+    use alloc::{borrow::ToOwned, boxed::Box, vec::Vec};
     use bevy_platform::sync::LazyLock;
+    use blocking::unblock;
     use std::io::{self, BufReader, Read};
 
     let str_path = path.to_str().ok_or_else(|| {