Make body stream writer sendable #125

Job	Run time
Cxx interop / Construct Cxx interop matrix	3s
Unit tests / Construct unit test matrix	5s
Soundness / API breakage check	4m 19s
Soundness / Documentation check	2m 52s
Soundness / Broken symlinks check	5s
Soundness / Format check	40s
Soundness / License headers check	5s
Soundness / Shell check	31s
Soundness / Unacceptable language check	2s
Soundness / Python lint check	9s
Soundness / YAML lint check	4s
Static SDK / Static SDK / Linux (latest-release Jammy)	3m 42s
Static SDK / Static SDK / Linux (main Jammy)	2m 8s
Cxx interop / Cxx interop / Linux (5.10)	2m 38s
Cxx interop / Cxx interop / Linux (6.0)	2m 54s
Cxx interop / Cxx interop / Linux (6.1)	2m 55s
Cxx interop / Cxx interop / Linux (nightly-next)	3m 36s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 38s
Unit tests / Unit tests / Linux (5.10)	4m 43s
Unit tests / Unit tests / Linux (6.0)	5m 8s
Unit tests / Unit tests / Linux (6.1)	5m 1s
Unit tests / Unit tests / Linux (nightly-next)	5m 42s
Unit tests / Unit tests / Linux (nightly-main)	6m 5s
	57m 5s

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