Make body stream writer sendable #123

Job	Run time
Cxx interop / Construct Cxx interop matrix	4s
Unit tests / Construct unit test matrix	3s
Soundness / Documentation check	2m 51s
Soundness / API breakage check	4m 18s
Soundness / Unacceptable language check	11s
Soundness / Broken symlinks check	2s
Soundness / Format check	38s
Soundness / License headers check	3s
Soundness / Shell check	40s
Soundness / Python lint check	18s
Soundness / YAML lint check	3s
Static SDK / Static SDK / Linux (latest-release Jammy)	3m 28s
Static SDK / Static SDK / Linux (main Jammy)	1m 57s
Unit tests / Unit tests / Linux (5.10)	4m 48s
Unit tests / Unit tests / Linux (6.0)	5m 20s
Unit tests / Unit tests / Linux (6.1)	4m 56s
Unit tests / Unit tests / Linux (nightly-next)	5m 52s
Unit tests / Unit tests / Linux (nightly-main)	5m 51s
Cxx interop / Cxx interop / Linux (5.10)	2m 36s
Cxx interop / Cxx interop / Linux (6.0)	2m 51s
Cxx interop / Cxx interop / Linux (6.1)	3m 1s
Cxx interop / Cxx interop / Linux (nightly-next)	3m 42s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 32s
	57m 5s

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