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.h

@@ -45,6 +45,11 @@ Stop the connection to the Sonar desktop.
 */
 - (void)stop;
 
+/**
+Get the current state of the sonar client
+*/
+- (NSString *)getState;
+
 // initializers are disabled. You must use `+[SonarClient sharedClient]` instance.
 - (instancetype)init NS_UNAVAILABLE;
 + (instancetype)new NS_UNAVAILABLE;