Add a separate trait for optional extractors #5055

Job	Run time
cargo-hack	5m 54s
check	1m 43s
cargo-public-api-crates (axum)	35s
check-docs	33s
cargo-public-api-crates (axum-core)	22s
dependencies-are-sorted	14s
Spell Check with Typos	0s
cargo-deny check (advisories)	25s
cargo-public-api-crates (axum-extra)	34s
cargo-deny check (bans licenses sources)	26s
cargo-public-api-crates (axum-macros)	17s
armv5te-unknown-linux-musleabi	1m 3s
test-nightly	1m 40s
test-docs	1m 51s
test-msrv	2m 31s
test-versions (stable)	3m 24s
wasm32-unknown-unknown	17s
test-versions (beta)	2m 39s
	24m 28s

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