It should be easier to fetch data from resources when constructing a scene

Goal:
I have a Resource containing Handles and I want to be able to create a Template fetching the Handles from the Resource which I can then pass into a Scene

```Rust
#[derive(Resource, Asset, Clone, TypePath)]
pub struct SomeAssets {
pub image1: Handle,
pub image2: Handle,
}

#[derive(Clone)]
pub enum SomeAssetTemplate {
Image1,
Image2,
}

impl Template for SomeAssetTemplate {
type Output = Handle;

fn build_template(&self, context: &mut TemplateContext) -> Result {
let some_assets = context.resource::();

match self {
SomeAssetTemplate::Image1=> Ok(tank_assets.image1.clone()),
SomeAssetTemplate::Image2=> Ok(tank_assets.image2.clone()),
}
}

fn clone_template(&self) -> Self {
self.clone()
}
}
```

it should be possible to now for example create a sprite with the new template somehow.

```
bsn! {
Sprite {
image: SomeAssetTemplate::Image1
}
}
```

discussion started here: https://discord.com/channels/691052431525675048/1518957482293334156

Start by reading the existing Template and TemplateContext APIs and the bsn! scene-construction syntax shown in the issue. Use the Discord discussion as context for the intended design. Done means a Resource's Handle fields can be fetched through a template and passed to a Sprite while constructing a Scene.

Written by the indexing model from the issue text.