Add a separate trait for optional extractors #5075

Job	Run time
check	42s
cargo-hack	2m 55s
cargo-deny check (advisories)	25s
cargo-public-api-crates (axum)	29s
dependencies-are-sorted	20s
cargo-deny check (bans licenses sources)	24s
check-docs	21s
cargo-public-api-crates (axum-core)	16s
cargo-public-api-crates (axum-extra)	33s
Spell Check with Typos	0s
cargo-public-api-crates (axum-macros)	17s
armv5te-unknown-linux-musleabi	29s
test-nightly	1m 6s
test-docs	1m 16s
test-msrv	1m 55s
test-versions (stable)	1m 20s
wasm32-unknown-unknown	18s
test-versions (beta)	1m 17s
	14m 23s

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