[Auth] Use result type to wrap completion handlers internally

[ncooke3]

Follow-up to #13561

Use Swift's Result type enum to prevent cases where a completion handler is called with both non-nil or both nil.

#no-changelog

[ncooke3]

This return type bubbles up to the public API, which we need to make non-optional if possible, so this is why this is being changed to non-optional String.

[ncooke3]

This diff should be a no-op in practice because the initializer will throw if it could not unwrap the value being assigned to responseInfo.

[ncooke3]

Ideally, this should be non-optional. I'm adding the error for cases where it's not. Are there success cases where verificationID should be nil?

[ncooke3]

Here and below, we need to unwrap the underlying session info which is current an optional:

firebase-ios-sdk/FirebaseAuth/Sources/Swift/Backend/RPC/Proto/Phone/AuthProtoStartMFAPhoneResponseInfo.swift

Lines 17 to 23 in 380a861

	struct AuthProtoStartMFAPhoneResponseInfo: AuthProto {
	var sessionInfo: String?
	init(dictionary: [String: AnyHashable]) {
	sessionInfo = dictionary["sessionInfo"] as? String
	}
	}

Throwing an error is new behavior here. I think this may be ok as long there aren't success cases where the sessionInfo should be nil?

[ncooke3]

The behavior changes made here map to this public API:

firebase-ios-sdk/FirebaseAuth/Sources/Swift/AuthProvider/PhoneAuthProvider.swift

Lines 97 to 109 in 380a861

	/// Verify ownership of the second factor phone number by the current user.
	/// - Parameter phoneNumber: The phone number to be verified.
	/// - Parameter uiDelegate: An object used to present the SFSafariViewController. The object is
	/// retained by this method until the completion block is executed.
	/// - Parameter multiFactorSession: A session to identify the MFA flow. For enrollment, this
	/// identifies the user trying to enroll. For sign-in, this identifies that the user already
	/// passed the first factor challenge.
	/// - Returns: The verification ID
	@available(iOS 13, tvOS 13, macOS 10.15, watchOS 8, *)
	open func verifyPhoneNumber(_ phoneNumber: String,
	uiDelegate: AuthUIDelegate? = nil,
	multiFactorSession: MultiFactorSession? = nil) async throws
	-> String {

Is there a valid success case where the returned String should be nil (or an empty string) and the error is also non-nil? This PR enforces that it is either one or the other (non-optional String OR error).