Add a separate trait for optional extractors (#2475) #5103

Job	Run time
check-docs	39s
check	1m 49s
cargo-hack	5m 5s
cargo-public-api-crates (axum)	44s
cargo-deny check (advisories)	25s
dependencies-are-sorted	21s
cargo-public-api-crates (axum-core)	21s
Spell Check with Typos	5s
cargo-deny check (bans licenses sources)	23s
cargo-public-api-crates (axum-extra)	32s
cargo-public-api-crates (axum-macros)	27s
test-docs	1m 38s
test-versions (stable)	3m 25s
test-msrv	2m 46s
armv5te-unknown-linux-musleabi	43s
test-nightly	1m 56s
test-versions (beta)	2m 56s
wasm32-unknown-unknown	23s
	24m 38s

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