axum: allow body types other than `axum::body::Body` in `Service`s passed to `serve` #5303

Job	Run time
check-docs	15s
check	38s
cargo-hack	3m 1s
cargo-public-api-crates (axum)	54s
cargo-deny check (advisories)	24s
cargo-public-api-crates (axum-core)	38s
dependencies-are-sorted	19s
cargo-deny check (bans licenses sources)	22s
Spell Check with Typos	4s
cargo-public-api-crates (axum-extra)	48s
cargo-public-api-crates (axum-macros)	40s
test-msrv	2m 39s
test-nightly	1m 10s
armv5te-unknown-linux-musleabi	21s
test-docs	1m 26s
test-versions (stable)	1m 16s
wasm32-unknown-unknown	14s
test-versions (beta)	1m 11s
	16m 20s

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