Add FLIPPER_PORTS env var to iOS SDK

Summary:
Part 2 of changing flipper to use customizable ports: iOS SDK.
Still to go: JS app.

env var should be of the format:

`FLIPPER_PORTS=1111,2222` for insecure port: 1111 and secure port: 2222.

Reviewed By: danielbuechele

Differential Revision: D13800988

fbshipit-source-id: 3560ba850389964f4b784551b803c45c6524e6f0

iOS/FlipperKit/SKEnvironmentVariables.h

@@ -0,0 +1,17 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the LICENSE
+ * file in the root directory of this source tree.
+ */
+#ifdef FB_SONARKIT_ENABLED
+
+/*
+ * This class exists to retreive configuration values stored in environment variables.
+ */
+@interface SKEnvironmentVariables : NSObject
+    + (int)getInsecurePort;
+    + (int)getSecurePort;
+@end
+
+#endif