make HTTPClient Sendable without @unchecked #157

Job	Run time
Unit tests / Construct unit test matrix	5s
Cxx interop / Construct Cxx interop matrix	6s
Release builds / Construct release build matrix	4s
Soundness / Unacceptable language check	6s
Soundness / License headers check	6s
Soundness / Broken symlinks check	5s
Soundness / YAML lint check	7s
Soundness / Format check	36s
Soundness / Shell check	39s
Soundness / Python lint check	7s
Soundness / API breakage check	4m 46s
Soundness / Documentation check	3m 5s
Static SDK / Static SDK / Linux (main Jammy)	4m 38s
Static SDK / Static SDK / Linux (latest-release Jammy)	3m 38s
Release builds / Release builds / Linux (6.1)	3m 59s
Release builds / Release builds / Linux (nightly-main)	5m 8s
Release builds / Release builds / Linux (nightly-next)	5m 21s
Release builds / Release builds / Linux (6.0)	4m 1s
Release builds / Release builds / Linux (5.10)	3m 38s
Unit tests / Unit tests / Linux (6.0)	5m 31s
Unit tests / Unit tests / Linux (6.1)	5m 24s
Unit tests / Unit tests / Linux (5.10)	4m 59s
Unit tests / Unit tests / Linux (nightly-main)	6m 11s
Unit tests / Unit tests / Linux (nightly-next)	6m 15s
Cxx interop / Cxx interop / Linux (6.1)	3m 17s
Cxx interop / Cxx interop / Linux (nightly-next)	3m 57s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 47s
Cxx interop / Cxx interop / Linux (6.0)	2m 57s
Cxx interop / Cxx interop / Linux (5.10)	2m 45s
	1h 25m 18s

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