iOS: introduce RCT_EXPORT_SYNCHRONOUS_TYPED_METHOD() macro to provide the return type of a sync method

Summary:
This provides a way to customize the return type of a sync exported method, instead of just using `id`.

```
RCT_EXPORT_SYNCHRONOUS_TYPED_METHOD(NSString *, sync1:(NSString *)x)
{
  return @"hello";
}
```

The return type needs to be a sub type of `id` - so scalars like `double` won't work (the bridge will crash).

Differential Revision: D6068884

fbshipit-source-id: 43a98141f1d0aef335aa0b33a24219f8e574e58b

React/Base/RCTBridgeModule.h

@@ -171,7 +171,11 @@ RCT_EXTERN void RCTRegisterModule(Class); \
  * is currently not supported.
  */
 #define RCT_EXPORT_BLOCKING_SYNCHRONOUS_METHOD(method) \
-  RCT_REMAP_BLOCKING_SYNCHRONOUS_METHOD(, method)
+  RCT_EXPORT_SYNCHRONOUS_TYPED_METHOD(id, method)
+
+#define RCT_EXPORT_SYNCHRONOUS_TYPED_METHOD(returnType, method) \
+  RCT_REMAP_BLOCKING_SYNCHRONOUS_METHOD(, returnType, method)
+
 
 /**
  * Similar to RCT_EXPORT_METHOD but lets you set the JS name of the exported
@@ -193,9 +197,9 @@ RCT_EXTERN void RCTRegisterModule(Class); \
  *   executeQuery:(NSString *)query parameters:(NSDictionary *)parameters)
  * { ... }
  */
-#define RCT_REMAP_BLOCKING_SYNCHRONOUS_METHOD(js_name, method) \
+#define RCT_REMAP_BLOCKING_SYNCHRONOUS_METHOD(js_name, returnType, method) \
   _RCT_EXTERN_REMAP_METHOD(js_name, method, YES) \
-  - (id)method;
+  - (returnType)method;
 
 /**
  * Use this macro in a private Objective-C implementation file to automatically