Make AsyncReadBody::with_capacity public #1010

Job	Run time
check-docs	1m 22s
cargo-hack	2m 35s
check	1m 37s
cargo-public-api-crates	1m 34s
cargo-deny check (advisories)	27s
cargo-deny check (bans licenses sources)	21s
style	12s
test-msrv	4m 53s
test-versions (stable)	3m 12s
test-versions (beta)	1m 47s
test-versions (nightly)	1m 41s
	19m 41s

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