Make axum generic over it's Listener type #4420

Job	Run time
check-docs	24s
cargo-hack	1m 7s
check	40s
cargo-public-api-crates (axum)	30s
cargo-deny check (advisories)	31s
dependencies-are-sorted	12s
cargo-deny check (bans licenses sources)	22s
cargo-public-api-crates (axum-core)	20s
cargo-public-api-crates (axum-extra)	27s
Spell Check with Typos	4s
cargo-public-api-crates (axum-macros)	15s
test-nightly	43s
test-docs	1m 53s
test-msrv	2m 43s
armv5te-unknown-linux-musleabi	45s
test-versions (stable)	2m 29s
wasm32-unknown-unknown	27s
test-versions (beta)	1m 16s
	15m 8s

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