Add async start hook to ExtensionApp API #2800

Job	Run time	Billable time
Test Docs	1m 43s	0s
Test Lint	1m 19s	0s
Test Examples	1m 8s	0s
build (ubuntu-latest, 3.8)	6m 25s	0s
Test Minimum Versions	4m 21s	0s
build (ubuntu-latest, 3.11)	5m 55s	0s
Test Prereleases	4m 2s	0s
build (windows-latest, 3.8)	13m 13s	0s
Check Links	3m 5s	0s
build (windows-latest, 3.11)	13m 32s	0s
build (macos-latest, 3.8)	6m 21s	0s
build (macos-latest, 3.11)	11m 15s	0s
check_release	1m 54s	0s
build (windows-latest, 3.9)	15m 56s	0s
build (ubuntu-latest, pypy-3.8)	10m 5s	0s
build (macos-latest, 3.10)	6m 51s	0s
integration_check (ubuntu-latest, 3.8)	2m 43s	0s
build (ubuntu-latest, 3.12)	6m 30s	0s
integration_check (ubuntu-latest, 3.9)	2m 27s	0s
integration_check (ubuntu-latest, 3.10)	2m 39s	0s
integration_check_pypy	2m 56s	0s
integration_check (ubuntu-latest, 3.11)	2m 18s	0s
Make SDist	30s	0s
Install from SDist and Test	4m 11s	0s
coverage	18s	0s
tests_check	3s	0s
	2h 11m 40s	0s

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