Move kGRPCErrorDomain to interface rather than Core implementation

src/objective-c/GRPCClient/GRPCCall.m

@@ -27,6 +27,8 @@
 NSString *const kGRPCHeadersKey = @"io.grpc.HeadersKey";
 NSString *const kGRPCTrailersKey = @"io.grpc.TrailersKey";
 
+NSString *const kGRPCErrorDomain = @"io.grpc";
+
 /**
  * The response dispatcher creates its own serial dispatch queue and target the queue to the
  * dispatch queue of a user provided response handler. It removes the requirement of having to use

src/objective-c/GRPCClient/private/GRPCCore/NSError+GRPC.m

@@ -19,8 +19,7 @@
 #import "NSError+GRPC.h"
 
 #include <grpc/grpc.h>
-
+#import <GRPCClient/GRPCTypes.h>
-NSString *const kGRPCErrorDomain = @"io.grpc";
 
 @implementation NSError (GRPC)
 + (instancetype)grpc_errorFromStatusCode:(grpc_status_code)statusCode