Make RequestBag conform to Sendable #131

Job	Run time
Cxx interop / Construct Cxx interop matrix	2s
Unit tests / Construct unit test matrix	3s
Soundness / Documentation check	2m 58s
Soundness / API breakage check	4m 26s
Soundness / License headers check	5s
Soundness / Unacceptable language check	2s
Soundness / Broken symlinks check	4s
Soundness / Format check	37s
Soundness / Python lint check	5s
Soundness / Shell check	35s
Soundness / YAML lint check	3s
Static SDK / Static SDK / Linux (latest-release Jammy)	3m 37s
Static SDK / Static SDK / Linux (main Jammy)	2m 14s
Cxx interop / Cxx interop / Linux (5.10)	2m 31s
Cxx interop / Cxx interop / Linux (6.0)	3m 4s
Cxx interop / Cxx interop / Linux (6.1)	2m 56s
Cxx interop / Cxx interop / Linux (nightly-next)	3m 31s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 38s
Unit tests / Unit tests / Linux (5.10)	4m 46s
Unit tests / Unit tests / Linux (6.0)	5m 25s
Unit tests / Unit tests / Linux (6.1)	4m 57s
Unit tests / Unit tests / Linux (nightly-next)	5m 41s
Unit tests / Unit tests / Linux (nightly-main)	5m 49s
	57m 9s

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