Prepare `serve` for potentially supporting graceful shutdown #4075

Job	Run time
cargo-hack	3m 32s
check	44s
cargo-public-api-crates (axum)	36s
check-docs	25s
cargo-public-api-crates (axum-core)	24s
cargo-deny check (advisories)	53s
cargo-public-api-crates (axum-extra)	30s
dependencies-are-sorted	24s
cargo-deny check (bans licenses sources)	23s
Spell Check with Typos	6s
cargo-public-api-crates (axum-macros)	14s
test-docs	1m 41s
test-msrv	1m 19s
armv5te-unknown-linux-musleabi	28s
test-nightly	1m 39s
test-versions (stable)	2m 14s
wasm32-unknown-unknown	28s
test-versions (beta)	1m 57s
	17m 57s

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