Implement `FromRequestParts` for `Parts` and `Extensions` #4020

Job	Run time
check	2m 7s
check-docs	50s
cargo-hack	6m 23s
cargo-public-api-crates (axum)	1m 6s
cargo-public-api-crates (axum-core)	45s
cargo-deny check (advisories)	27s
dependencies-are-sorted	41s
cargo-public-api-crates (axum-extra)	55s
cargo-deny check (bans licenses sources)	24s
Spell Check with Typos	4s
cargo-public-api-crates (axum-macros)	46s
test-msrv	6m 57s
test-nightly	2m 3s
wasm32-unknown-unknown	33s
test-docs	2m 7s
test-versions (stable)	2m 23s
test-versions (beta)	4m 16s
armv5te-unknown-linux-musleabi	48s
	33m 35s

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