Make body stream writer sendable #124

Job	Run time
Unit tests / Construct unit test matrix	5s
Cxx interop / Construct Cxx interop matrix	2s
Soundness / API breakage check	4m 23s
Soundness / Broken symlinks check	8s
Soundness / Documentation check	2m 49s
Soundness / Format check	42s
Soundness / License headers check	6s
Soundness / Python lint check	4s
Soundness / Shell check	30s
Soundness / Unacceptable language check	5s
Soundness / YAML lint check	4s
Static SDK / Static SDK / Linux (latest-release Jammy)	3m 24s
Static SDK / Static SDK / Linux (main Jammy)	1m 58s
Cxx interop / Cxx interop / Linux (5.10)	2m 33s
Cxx interop / Cxx interop / Linux (6.0)	2m 55s
Cxx interop / Cxx interop / Linux (6.1)	2m 56s
Cxx interop / Cxx interop / Linux (nightly-next)	3m 32s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 42s
Unit tests / Unit tests / Linux (5.10)	4m 49s
Unit tests / Unit tests / Linux (6.0)	5m 15s
Unit tests / Unit tests / Linux (6.1)	5m 5s
Unit tests / Unit tests / Linux (nightly-next)	5m 50s
Unit tests / Unit tests / Linux (nightly-main)	6m 6s
	57m 3s

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