Make GRPCRequestHeaders public and import from GRPCCall

src/objective-c/GRPCClient/GRPCCall+OAuth2.m

@@ -32,7 +32,6 @@
  */
 
 #import "GRPCCall+OAuth2.h"
-#import "private/GRPCRequestHeaders.h"
 
 static NSString * const kAuthorizationHeader = @"authorization";
 static NSString * const kBearerPrefix = @"Bearer ";

src/objective-c/GRPCClient/GRPCCall.h

@@ -48,7 +48,7 @@
 #import <Foundation/Foundation.h>
 #import <RxLibrary/GRXWriter.h>
 
-@class GRPCRequestHeaders;
+#import "GRPCRequestHeaders.h"
 
 // Keys used in |NSError|'s |userInfo| dictionary to store the response headers and trailers sent by
 // the server.

src/objective-c/GRPCClient/GRPCCall.m

@@ -41,7 +41,6 @@
 #import "private/NSData+GRPC.h"
 #import "private/NSDictionary+GRPC.h"
 #import "private/NSError+GRPC.h"
-#import "private/GRPCRequestHeaders.h"
 
 NSString * const kGRPCHeadersKey = @"io.grpc.HeadersKey";
 NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";