feat(core): Add Sentry.appLoaded() API to signal app start end (#5940) #12796

Job	Run time
Ready-to-merge gate / Missing "ready-to-merge" label	2s
diff_check / diff_check	2s
detect-changes / Detect file changes	4s
Build legacy macos dev	5m 20s
Build new android production	12m 39s
Build legacy android production	10m 2s
Build legacy macos production	5m 34s
Build legacy ios dev dynamic-frameworks	9m 37s
Build new ios production no-frameworks	10m 0s
Build new android dev	10m 12s
Build legacy ios dev no-frameworks	8m 30s
Build legacy ios production no-frameworks	9m 18s
Build legacy android dev	9m 28s
Build new ios dev no-frameworks	9m 40s
Build legacy ios production dynamic-frameworks	8m 56s
Test android production REV2	6m 12s
Test ios production REV2	4m 22s
	1h 59m 58s

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