feat: Add Rokt close implementation (#19)

Add close implementation

Author: jamesnrokt

mParticle-Rokt/MPKitRokt.m

@@ -415,6 +415,11 @@ - (MPKitExecStatus *)events:(NSString *)identifier onEvent:(void (^)(MPRoktEvent
     return [[MPKitExecStatus alloc] initWithSDKCode:[[self class] kitCode] returnCode:MPKitReturnCodeSuccess];
 }
 
+- (MPKitExecStatus *)close {
+    [Rokt close];
+    return [[MPKitExecStatus alloc] initWithSDKCode:[[self class] kitCode] returnCode:MPKitReturnCodeSuccess];
+}
+
 #pragma mark - User attributes and identities
 
 - (MPKitExecStatus *)setUserIdentity:(NSString *)identityString identityType:(MPUserIdentity)identityType {