Add description to OSRequestGetInAppMessages

* Without a description, the object will return something like `<OSRequestGetInAppMessages: 0x600001798bc0>`. By adding a predictable description, we can operate on the object.

Author: nan-li

iOS_SDK/OneSignalSDK/OneSignalInAppMessages/Requests/OSInAppMessagingRequests.m

@@ -28,6 +28,10 @@ of this software and associated documentation files (the "Software"), to deal
 #import "OSInAppMessagingRequests.h"
 
 @implementation OSRequestGetInAppMessages
+- (NSString *)description {
+    return [NSString stringWithFormat:@"<OSRequestGetInAppMessages from %@>", self.path];
+}
+
 + (instancetype _Nonnull)   withSubscriptionId:(NSString * _Nonnull)subscriptionId
                             withSessionDuration:(NSNumber * _Nonnull)sessionDuration
                             withRetryCount:(NSNumber *)retryCount