[Auth] TOTP support for macOS

[cbenhagen]

This PR adds support for TOTP on macOS.

Partially fixes #10034

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[ncooke3]

Thanks for the PR, @cbenhagen! Please fix the style check: https://github.com/firebase/firebase-ios-sdk/actions/runs/16248298480/job/45968757691?pr=15112

See https://github.com/firebase/firebase-ios-sdk?tab=readme-ov-file#code-formatting for the prereqs for running the scripts/style.sh.

[cbenhagen]

@ncooke3 sorry for the oversight! Updated my branch.

[cbenhagen]

I also added FIRTOTPSecret.h to make sure the Flutter plugin compiles. Please let me know if more headers are needed.

[ncooke3]

@cbenhagen, were you able to do an E2E test of this feature?

[cbenhagen]

@cbenhagen, were you able to do an E2E test of this feature?

@ncooke3, yes I tested this with our Flutter app and am able to enroll & unenroll as well as login with the TOTP factor.

[ncooke3]

Thanks again, @cbenhagen!