feat(core): Add Sentry.appLoaded() API to signal app start end #12795

Job	Run time
Ready-to-merge gate / Missing "ready-to-merge" label	4s
diff_check / diff_check	9s
detect-changes / Detect file changes	7s
Build legacy macos production	5m 48s
Build new android dev	10m 17s
Build legacy macos dev	5m 31s
Build legacy android production	9m 51s
Build legacy ios production no-frameworks	8m 24s
Build legacy android dev	8m 14s
Build legacy ios production dynamic-frameworks	20m 7s
Build new ios dev no-frameworks	9m 39s
Build new android production	12m 9s
Build new ios production no-frameworks	10m 33s
Build legacy ios dev no-frameworks	8m 31s
Build legacy ios dev dynamic-frameworks	8m 52s
Test android production REV2	5m 29s
Test ios production REV2	1s
	2h 3m 46s

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