More Xcode 15 CI

[paulb777]

Add Xcode 15 to pod lib lint and spm CI for functions, fiam, installations, mlmodeldownloader, performance, remoteconfig, sessions, shared-swift and spm.

I disabled some tests on Xcode 15 because of iOS simulator flakiness and will monitor upcoming nightlies for the need to do more.

Also, fixed a nullability issue in a FIAM internal API and a FIAM unit test failure triggered by the recent changes in handling URL strings.

I disabled a Performance unit test that failed on SPM in addition to the several more that were already disabled for SPM. See #11903

#no-changelog

[google-oss-bot]

Coverage Report ¹

Affected Products

• FirebaseMessaging-iOS-FirebaseMessaging.framework

  Overall coverage changed from 60.56% (fa7ca72) to 60.59% (1106397) by +0.03%.

  Filename	Base (fa7ca72)	Merge (1106397)	Diff
  FIRMessagingAnalytics.m	80.00%	82.14%	+2.14%
  FIRMessagingCheckinPreferences.m	98.02%	93.07%	-4.95%
  FIRMessagingPendingTopicsList.m	89.02%	90.59%	+1.57%

• FirebasePerformance-iOS-FirebasePerformance.framework

  Overall coverage changed from 94.71% (fa7ca72) to 94.74% (1106397) by +0.03%.

  Filename	Base (fa7ca72)	Merge (1106397)	Diff
  FPRNetworkTrace.m	95.25%	95.85%	+0.59%

Test Logs

• Base (fa7ca72): https://github.com/firebase/firebase-ios-sdk/actions/runs/6736260448
• Merge (1106397): https://github.com/firebase/firebase-ios-sdk/actions/runs/6748476043

1. https://storage.googleapis.com/firebase-sdk-metric-reports/nLwgukaWIF.html

[andrewheard]

Thanks, Paul!