Expose iOS MultiFactorResolver to FirebaseAuthMultiFactorException

[RaphaelHx]

When a user has been enrolled into multifactor, on sign in, a FirebaseAuthMultiFactorException is thrown, and should contain a MultiFactorResolver. This has a function resolveSignIn to handle entering the code.

On Android for example, the exception contains the resolver, and can be extracted and used successfully.

import com.google.firebase.auth.FirebaseAuthMultiFactorException
import dev.gitlive.firebase.auth.MultiFactorResolver

actual fun extractMultiFactorResolver(e: FirebaseAuthMultiFactorException): MultiFactorResolver? {
    return MultiFactorResolver(e.resolver)
}

https://firebase.google.com/docs/auth/android/totp-mfa#sign_in_users_with_a_second_factor

However, on iOS, the resolver isn't included in the exception, and so the code below wouldn't work.

import dev.gitlive.firebase.auth.FirebaseAuthMultiFactorException
import dev.gitlive.firebase.auth.MultiFactorResolver
import kotlinx.cinterop.ExperimentalForeignApi

@OptIn(ExperimentalForeignApi::class)
actual fun extractMultiFactorResolver(e: FirebaseAuthMultiFactorException): MultiFactorResolver? {
    val resolver = e.resolver ?: return null
    return MultiFactorResolver(resolver)
}

https://firebase.google.com/docs/auth/ios/totp-mfa#sign_in_users_with_a_second_factor

This is because NSError.toException() function throws a new error without exposing the required resolver.

17087L, // AuthErrorCode.secondFactorAlreadyEnrolled
17078L, // AuthErrorCode.secondFactorRequired
17088L, // AuthErrorCode.maximumSecondFactorCountExceeded
17084L, // AuthErrorCode.multiFactorInfoNotFound
-> FirebaseAuthMultiFactorException(toString())

Adding the resolver in the exception allows iOS to complete sign in for users with multi factor enrolled.

[RaphaelHx]

Fixes #727

[RaphaelHx]

Matches the behaviour of android

[RaphaelHx]

@nbransby @Daeda88 tagging to put this on your radar, thank you.