Add arbitrary request headers to EhttpLoader (#8121)

* Closes <https://github.com/emilk/egui/issues/4491>
* [x] I have followed the instructions in the PR template

Lets you create an `EhttpLoader` with arbitrary headers like so:

```rust
cc.egui_ctx.add_bytes_loader(std::sync::Arc::new(
    egui_extras::loaders::http_loader::EhttpLoader::default().with_headers(&[
        ("User-Agent", "foo"),
    ])
));
```

I'm not sure if there are any problems with installing a second
`EhttpLoader` (in addition to the one installed by default when using
`egui_extras::install_image_loaders(&cc.egui_ctx)`. But I wasn't
sure how else to pass in configuration options to
`install_image_loaders`.

crates/egui_extras/src/loaders/http_loader.rs

@@ -42,10 +42,23 @@ type Entry = Poll<Result<File, String>>;
 #[derive(Default)]
 pub struct EhttpLoader {
     cache: Arc<Mutex<HashMap<String, Entry>>>,
+    request_template: Option<Box<dyn Fn(ehttp::Request) -> ehttp::Request + Send + Sync>>,
 }
 
 impl EhttpLoader {
     pub const ID: &'static str = egui::generate_loader_id!(EhttpLoader);
+
+    /// Provide a request template to modify requests before they're sent,
+    /// e.g. to add headers.
+    pub fn with_request_template<
+        F: Fn(ehttp::Request) -> ehttp::Request + Send + Sync + 'static,
+    >(
+        mut self,
+        request_template: F,
+    ) -> Self {
+        self.request_template = Some(Box::new(request_template));
+        self
+    }
 }
 
 const PROTOCOLS: &[&str] = &["http://", "https://"];
@@ -82,7 +95,12 @@ impl BytesLoader for EhttpLoader {
             cache.insert(uri.clone(), Poll::Pending);
             drop(cache);
 
-            ehttp::fetch(ehttp::Request::get(uri.clone()), {
+            ehttp::fetch(
+                match &self.request_template {
+                    Some(templ) => templ(ehttp::Request::get(uri.clone())),
+                    None => ehttp::Request::get(uri.clone()),
+                },
+                {
                     let ctx = ctx.clone();
                     let cache = Arc::clone(&self.cache);
                     move |response| {
@@ -116,7 +134,8 @@ impl BytesLoader for EhttpLoader {
                             ctx.request_repaint();
                         }
                     }
-            });
+                },
+            );
 
             Ok(BytesPoll::Pending { size: None })
         }