Entirely control Flipper being enabled through Podfile (#1086)

Summary:
Currently user’s are being told to add a definition of the `FB_SONARKIT_ENABLED` macro and examples, including those in stock React Native templates, set this for the user by making use of a `post_install` hook in the user’s `Podfile`. This leads to confusion, fragile code [when a user’s project dir structure deviates from vanilla], and is ultimately not necessary as CocoaPods already has dedicated mechanisms to:

* specify build settings (through the `xcconfig` property);
* and selectively include certain pods only in certain build configurations (e.g. debug).

Finally, this PR also includes a commit [to fix the current builds](https://github.com/facebook/flipper/pull/1086/files#r418526812).

## Changelog

> Entirely control Flipper being enabled through inclusion in Podfile and optionally limiting to certain build configurations using the `:configuration` directive.
Pull Request resolved: https://github.com/facebook/flipper/pull/1086

Test Plan: I have built and ran the Sample application, as well as used this version of Flipper with a new RN app built from `master`.

Reviewed By: passy

Differential Revision: D21381828

Pulled By: priteshrnandgaonkar

fbshipit-source-id: edf6dae28eb02336a49e8230654d6186360ea8d6

iOS/Plugins/FlipperKitLayoutPlugin/FlipperKitLayoutComponentKitSupport/SKComponentLayoutWrapper.h

@@ -10,7 +10,7 @@
 #import <ComponentKit/CKComponentLayout.h>
 #import <ComponentKit/CKFlexboxComponent.h>
 #import <ComponentKit/CKOptional.h>
-#import <ComponentKit/CKVariant.h>
+#import <RenderCore/CKVariant.h>
 
 #import <vector>