[Swift 6] Add AtomicBox to CoreInternal

[ncooke3]

Plan: Add AtomicBox to each module as needed as part of addressing Swift 6 warnings. Then, when we move to Xcode 16, delete all definitions of AtomicBox apart from the one defined in this PR, in FirebaseCoreInternal. Reason being is that Xcode 16 gives us Swift 6 support for internal import FirebaseCoreInternal in other product SDKs that can then get AtomicBox.

#no-changelog

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