feat(core): allow swapping the assets implementation

[lucasfernog]

The current Context::assets_mut is useless since you must provide a type that matches the generic arg in Context, so we're dropping it in favor of a Box instead.

Additionally, during codegen you can now define your custom assets implementation (by default we embed the frontendDist assets):

struct MyAssets {
...
}
impl tauri::Assets for MyAssets {
...
}

fn main() {
  let assets = MyAssets {};
  tauri::generate_context(
    assets = assets
  );
}

[amrbashir]

any reason not to return &dyn Assets here?

[lucasfernog]

oops forgot i could use as_ref, gonna push