Use async-fn-in-trait (RPIT) for various Asset traits. #34748

Job	Run time
build (windows-latest)	2m 33s
build (ubuntu-latest)	2m 29s
build (macos-latest)	2m 41s
check-bevy-internal-imports	4s
check-doc	6m 34s
check-missing-examples-in-docs	27s
ci	21s
miri	7m 31s
run-examples-on-windows-dx12	17m 17s
toml	5s
check-compiles	0s
check-missing-features-in-docs	39s
markdownlint	1m 10s
build-wasm	0s
msrv	0s
	41m 51s

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