Fix usage of the library in extensions to reflect changes in XCode >= 13

[jefmathiot]

Summary

Starting with XCode 13 beta 3, usage of "unsafe" code in application extensions is forbidden and results in build errors. Linked Swift packages are expected to be annotated to explicitely declare what code is unavailable in extensions.

Motivation

Fix the build when using Stripe iOS SDK with Xcode >= 13 beta 3 :)

Testing

Real world testing on an Issuing push provisioning integration of Stripe SDK, with XCode 15.4.

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