Use async-fn-in-trait (RPIT) for various Asset traits. #36040

Job	Run time
build (windows-latest)	7m 41s
check-doc	6m 31s
build (ubuntu-latest)	3m 16s
build (macos-latest)	6m 8s
check-missing-examples-in-docs	29s
ci	1m 43s
miri	7m 5s
run-examples-on-windows-dx12	14m 14s
toml	4s
check-bevy-internal-imports	4s
check-missing-features-in-docs	29s
markdownlint	1m 9s
check-compiles	0s
msrv	1m 23s
build-wasm	1m 28s
	51m 44s

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