feat: add `WindowBuilder::on_download`, closes #8157 #8810

Job	Run time
test (x86_64-pc-windows-msvc, windows-latest, 1.70.0, false, test, --no-default-features, no-defa...	3m 40s
test (x86_64-pc-windows-msvc, windows-latest, 1.70.0, false, test, --all-features, all)	3m 33s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.70.0, false, test, --no-default-features, no-def...	4m 25s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.70.0, false, test, --all-features, all)	6m 0s
test (x86_64-apple-darwin, macos-latest, 1.70.0, false, test, --no-default-features, no-default)	5m 59s
test (x86_64-apple-darwin, macos-latest, 1.70.0, false, test, --all-features, all)	8m 16s
test (aarch64-apple-ios, macos-latest, 1.70.0, false, build, --no-default-features, no-default)	2m 14s
test (aarch64-apple-ios, macos-latest, 1.70.0, false, build, --all-features, all)	2m 4s
test (aarch64-linux-android, ubuntu-latest, 1.70.0, true, build, --no-default-features, no-default)	2m 36s
test (aarch64-linux-android, ubuntu-latest, 1.70.0, true, build, --all-features, all)	2m 13s
	41m 0s

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