Make body stream writer sendable #122

Job	Run time
Cxx interop / Construct Cxx interop matrix	4s
Unit tests / Construct unit test matrix	3s
Soundness / API breakage check	4m 19s
Soundness / Broken symlinks check	4s
Soundness / Documentation check	2m 56s
Soundness / License headers check	5s
Soundness / Unacceptable language check	3s
Soundness / Format check	35s
Soundness / Python lint check	5s
Soundness / Shell check	33s
Soundness / YAML lint check	5s
Static SDK / Static SDK / Linux (latest-release Jammy)	3m 31s
Static SDK / Static SDK / Linux (main Jammy)	1m 40s
Unit tests / Unit tests / Linux (5.10)	4m 42s
Unit tests / Unit tests / Linux (6.0)	5m 10s
Unit tests / Unit tests / Linux (6.1)	5m 4s
Unit tests / Unit tests / Linux (nightly-next)	5m 47s
Unit tests / Unit tests / Linux (nightly-main)	5m 54s
Cxx interop / Cxx interop / Linux (5.10)	2m 34s
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 34s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 37s
	56m 14s

Provide feedback