[nit] Break up User Executor code

* swiftlint error: Type Body Length Violation: Type body should span 350 lines or less excluding comments and whitespace: currently spans 354 lines (type_body_length)
* No logic changes in this commit, only split up executing requests code into an extension

Author: nan-li

iOS_SDK/OneSignalSDK/OneSignalUser/Source/Executors/OSUserExecutor.swift

@@ -196,7 +196,10 @@ class OSUserExecutor {
             }
         }
     }
+}
 
+// MARK: - Execution
+extension OSUserExecutor {
     // We will pass minimal properties to this request
     static func createUser(_ user: OSUserInternal) {
         let originalPushToken = user.pushSubscriptionModel.address