Support adding services while `ServiceGroup` is running #29

Job	Run time
Unit tests / Construct unit test matrix	2s
Cxx interop / Construct Cxx interop matrix	2s
Soundness / API breakage check	1m 2s
Soundness / Documentation check	1m 13s
Soundness / Broken symlinks check	5s
Soundness / License headers check	5s
Soundness / Unacceptable language check	3s
Soundness / Python lint check	5s
Soundness / Format check	32s
Soundness / Shell check	34s
Soundness / YAML lint check	5s
Unit tests / Unit tests / Linux (5.9)	48s
Unit tests / Unit tests / Linux (5.10)	47s
Unit tests / Unit tests / Linux (6.0)	59s
Unit tests / Unit tests / Linux (nightly-next)	1m 3s
Unit tests / Unit tests / Linux (nightly-main)	1m 4s
Cxx interop / Cxx interop / Linux (5.9)	47s
Cxx interop / Cxx interop / Linux (5.10)	47s
Cxx interop / Cxx interop / Linux (6.0)	57s
Cxx interop / Cxx interop / Linux (nightly-next)	1m 6s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 0s
	13m 6s

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