Add a separate trait for optional extractors #5076

Job	Run time
check-docs	20s
cargo-public-api-crates (axum)	30s
cargo-hack	2m 53s
cargo-public-api-crates (axum-core)	15s
check	54s
cargo-deny check (advisories)	26s
cargo-public-api-crates (axum-extra)	27s
dependencies-are-sorted	15s
cargo-deny check (bans licenses sources)	32s
cargo-public-api-crates (axum-macros)	20s
Spell Check with Typos	0s
test-nightly	1m 8s
test-docs	1m 15s
armv5te-unknown-linux-musleabi	28s
test-msrv	1m 49s
test-versions (stable)	1m 16s
wasm32-unknown-unknown	23s
test-versions (beta)	1m 19s
	14m 30s

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