Add a separate trait for optional extractors #5063

Job	Run time
check	1m 47s
check-docs	24s
cargo-public-api-crates (axum)	31s
cargo-hack	5m 37s
cargo-public-api-crates (axum-core)	18s
cargo-deny check (advisories)	25s
dependencies-are-sorted	17s
cargo-public-api-crates (axum-extra)	36s
cargo-deny check (bans licenses sources)	28s
Spell Check with Typos	0s
cargo-public-api-crates (axum-macros)	17s
test-docs	1m 44s
test-nightly	1m 35s
armv5te-unknown-linux-musleabi	56s
test-msrv	1m 54s
test-versions (stable)	3m 34s
wasm32-unknown-unknown	27s
test-versions (beta)	2m 34s
	23m 24s

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