iOS: Add very basic diagnostic screen

Summary:
Adds an extremely barebones in-app screen for showing sonar diagnostics.
The UI is laughable, but it's wired up. Leaving proper design to a later commit.

Like in the android case, state is currently passed as a string, but will become something much more structured after this is in place.

Reviewed By: priteshrnandgaonkar

Differential Revision: D9218766

fbshipit-source-id: 4889ed79b928056a67b1e8cb8a40a9bd52e084f1

iOS/SonarKit/SonarClient.mm

@@ -121,6 +121,10 @@ using WrapperPlugin = facebook::sonar::SonarCppWrapperPlugin;
 #endif
 }
 
+- (NSString *)getState {
+  return @(_cppClient->getState().c_str());
+}
+
 @end
 
 #endif