Implement `Handler` for `T: IntoResponse` #3798

Job	Run time
cargo-hack	5m 45s
check	1m 42s
cargo-public-api-crates (axum)	1m 1s
cargo-public-api-crates (axum-core)	26s
cargo-public-api-crates (axum-extra)	1m 0s
cargo-public-api-crates (axum-macros)	22s
check-docs	20s
cargo-deny check (advisories)	39s
cargo-deny check (bans licenses sources)	33s
dependencies-are-sorted	24s
test-msrv	2m 4s
test-docs	3m 38s
armv5te-unknown-linux-musleabi	48s
test-versions (stable)	3m 10s
test-versions (beta)	3m 22s
test-nightly	3m 59s
wasm32-unknown-unknown	15s
	29m 28s

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