Add example showing how to proxy `reqwest::Response` #3793

Job	Run time
cargo-hack	5m 28s
check	1m 13s
check-docs	40s
cargo-public-api-crates (axum)	54s
cargo-public-api-crates (axum-core)	28s
cargo-public-api-crates (axum-extra)	58s
cargo-public-api-crates (axum-macros)	29s
cargo-deny check (advisories)	33s
cargo-deny check (bans licenses sources)	50s
dependencies-are-sorted	26s
test-nightly	2m 49s
test-versions (stable)	3m 15s
test-versions (beta)	3m 27s
test-docs	3m 44s
armv5te-unknown-linux-musleabi	1m 8s
test-msrv	2m 22s
wasm32-unknown-unknown	30s
	29m 14s

Provide feedback