MFA Integration Testing fixes

[paulb777]

Fix four issues integration testing Multi-Factor Authentication thanks to the test case from #11079

• Build issue from using withCompletion: instead of completion: in some APIs
• Created an AuthMFAResponse protocol for responses that need AuthBackend's checking of MFA error throwing
• Fixed the type of MultiFactorInfo hint arrays. They should be PhoneMultiFactorInfo and TOTPMultiFactorInfo instead of the common base type.
• Fixed a logic error in PhoneAuthProvider.swift: idToken can be non-nil for both the recaptcha and credential cases.

[google-oss-bot]

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