Use `async fn` in trait instead of `BoxedFuture`

[tguichaoua]

What problem does this solve or what need does it fill?

async fn and return-position impl Trait in trait as been stabilized in 1.75 (rust-lang/rust#115822)

What solution would you like?

Replace usage of BoxedFuture in traits like AssetLoader by async fn.

bevy/crates/bevy_asset/src/loader.rs

Lines 33 to 38 in ce5bae5

	fn load<'a>(
	&'a self,
	reader: &'a mut Reader,
	settings: &'a Self::Settings,
	load_context: &'a mut LoadContext,
	) -> BoxedFuture<'a, Result<Self::Asset, Self::Error>>;

 async fn load<'a>( 
     &'a self, 
     reader: &'a mut Reader, 
     settings: &'a Self::Settings, 
     load_context: &'a mut LoadContext, 
 ) -> Result<Self::Asset, Self::Error>; 

What alternative(s) have you considered?

Keep using BoxedFuture with Box::pin(async move { /* */ });.

Current limitations

Because of the send bound problem, there is no way, yet, to add the Send bound on the returned future.
Alternatively it's possible to use impl Future + Send:

 fn load<'a>( 
     &'a self, 
     reader: &'a mut Reader, 
     settings: &'a Self::Settings, 
     load_context: &'a mut LoadContext, 
 ) -> impl Future<Output = Result<Self::Asset, Self::Error>> + Send + 'a; 

[james7132]

This was implemented in #12550 and follow-up PRs.