feat(core): Add Sentry.appLoaded() API to signal app start end #12794

Job	Run time
Ready-to-merge gate / Missing "ready-to-merge" label	3s
diff_check / diff_check	9s
detect-changes / Detect file changes	4s
Build legacy macos production	6m 27s
Build legacy macos dev	2m 58s
Build new android dev	8m 44s
Build legacy ios dev no-frameworks	8m 41s
Build legacy android dev	8m 44s
Build legacy ios dev dynamic-frameworks	8m 9s
Build new android production	8m 43s
Build new ios production no-frameworks	8m 42s
Build new ios dev no-frameworks	6m 28s
Build legacy android production	8m 43s
Build legacy ios production dynamic-frameworks	8m 36s
Build legacy ios production no-frameworks	4m 49s
Test android production REV2	1s
Test ios production REV2	1s
	1h 30m 2s

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