Add GltfLoader::new. (#9120)

# Objective

In my application, I'm manually wrapping the built-in Bevy loaders with
a wrapper loader that stores some metadata before calling into the inner
Bevy loader. This worked for the glTF loader in Bevy 0.10, but in Bevy
0.11 it became impossible to do this because the glTF loader became
unconstructible outside Bevy due to the new private fields within it.
It's now in fact impossible to get a reference to a GltfLoader at all
from outside Bevy, because the only way to construct a GltfLoader is to
add the GltfPlugin to an App, and the GltfPlugin only hands out
references to its GltfLoader to the asset server, which provides no
public access to the loaders it manages.

## Solution

This commit fixes the problem by adding a public `new` method to allow
manual construction of a glTF loader.

---------

Co-authored-by: Carter Anderson <mcanders1@gmail.com>

crates/bevy_gltf/src/loader.rs

@@ -70,8 +70,8 @@ pub enum GltfError {
 
 /// Loads glTF files with all of their data as their corresponding bevy representations.
 pub struct GltfLoader {
-    pub(crate) supported_compressed_formats: CompressedImageFormats,
-    pub(crate) custom_vertex_attributes: HashMap<String, MeshVertexAttribute>,
+    pub supported_compressed_formats: CompressedImageFormats,
+    pub custom_vertex_attributes: HashMap<String, MeshVertexAttribute>,
 }
 
 impl AssetLoader for GltfLoader {