Add a separate trait for optional extractors #5102

Job	Run time
check	1m 41s
cargo-hack	4m 30s
check-docs	20s
cargo-public-api-crates (axum)	40s
cargo-public-api-crates (axum-core)	28s
cargo-deny check (advisories)	24s
dependencies-are-sorted	14s
cargo-public-api-crates (axum-extra)	34s
Spell Check with Typos	5s
cargo-deny check (bans licenses sources)	26s
cargo-public-api-crates (axum-macros)	25s
test-versions (stable)	2m 34s
test-versions (beta)	2m 33s
test-msrv	2m 28s
armv5te-unknown-linux-musleabi	32s
test-docs	1m 34s
test-nightly	1m 34s
wasm32-unknown-unknown	12s
	21m 14s

Provide feedback