Add GltfLoaderSettings (#10804)

# Objective

when loading gltfs we may want to filter the results. in particular, i
need to be able to exclude cameras.

i can do this already by modifying the gltf after load and before
spawning, but it seems like a useful general option.

## Solution

add `GltfLoaderSettings` struct with bool members:
- `load_cameras` : checked before processing camera nodes.
- `load_lights` : checked before processing light nodes
- `load_meshes` : checked before loading meshes, materials and morph
weights

Existing code will work as before. Now you also have the option to
restrict what parts of the gltf are loaded. For example, to load a gltf
but exclude the cameras, replace a call to
`asset_server.load("my.gltf")` with:
```rust
asset_server.load_with_settings(
    "my.gltf",
    |s: &mut GltfLoaderSettings| {
        s.load_cameras = false;
    }
);
```

---------

Co-authored-by: Alice Cecile <[email protected]>

Author: robtfm