[Auth] Update WithdrawMFARequest to have only immutable properties

Author: ncooke3

FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/Unenroll/WithdrawMFARequest.swift

@@ -23,8 +23,8 @@ private let kTenantIDKey = "tenantId"
 class WithdrawMFARequest: IdentityToolkitRequest, AuthRPCRequest {
   typealias Response = WithdrawMFAResponse
 
-  var idToken: String?
-  var mfaEnrollmentID: String?
+  let idToken: String?
+  let mfaEnrollmentID: String?
 
   init(idToken: String?,
        mfaEnrollmentID: String?,