Publish GRPCRequestHeaders as a protocol

So we can make the property a dictionary later, by just extending
NSMutableDictionary to conform to the protocol.

src/objective-c/GRPCClient/GRPCCall.h

@@ -48,13 +48,26 @@
 #import <Foundation/Foundation.h>
 #import <RxLibrary/GRXWriter.h>
 
-#import "GRPCRequestHeaders.h"
+#include <grpc/grpc.h>
 
 // Keys used in |NSError|'s |userInfo| dictionary to store the response headers and trailers sent by
 // the server.
 extern id const kGRPCHeadersKey;
 extern id const kGRPCTrailersKey;
 
+@protocol GRPCRequestHeaders <NSObject>
+
+@property(nonatomic, readonly) NSUInteger count;
+@property(nonatomic, readonly) grpc_metadata *grpc_metadataArray;
+
+- (id)objectForKeyedSubscript:(NSString *)key;
+- (void)setObject:(id)obj forKeyedSubscript:(NSString *)key;
+
+- (void)removeAllObjects;
+- (void)removeObjectForKey:(NSString *)key;
+
+@end
+
 // Represents a single gRPC remote call.
 @interface GRPCCall : GRXWriter
 
@@ -72,7 +85,7 @@ extern id const kGRPCTrailersKey;
 //
 // For convenience, the property is initialized to an empty NSMutableDictionary, and the setter
 // accepts (and copies) both mutable and immutable dictionaries.
-- (GRPCRequestHeaders *)requestHeaders; // nonatomic
+- (id<GRPCRequestHeaders>)requestHeaders; // nonatomic
 - (void)setRequestHeaders:(NSDictionary *)requestHeaders; // nonatomic, copy
 
 // This dictionary is populated with the HTTP headers received from the server. This happens before

src/objective-c/GRPCClient/GRPCCall.m

@@ -37,6 +37,7 @@
 #include <grpc/support/time.h>
 #import <RxLibrary/GRXConcurrentWriteable.h>
 
+#import "private/GRPCRequestHeaders.h"
 #import "private/GRPCWrappedCall.h"
 #import "private/NSData+GRPC.h"
 #import "private/NSDictionary+GRPC.h"
@@ -131,7 +132,7 @@ NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";
 
 #pragma mark Metadata
 
-- (GRPCRequestHeaders *)requestHeaders {
+- (id<GRPCRequestHeaders>)requestHeaders {
   return _requestHeaders;
 }
 
@@ -234,7 +235,7 @@ NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";
 
 #pragma mark Send headers
 
-- (void)sendHeaders:(GRPCRequestHeaders *)headers {
+- (void)sendHeaders:(id<GRPCRequestHeaders>)headers {
   // TODO(jcanizales): Add error handlers for async failures
   [_wrappedCall startBatchWithOperations:@[[[GRPCOpSendMetadata alloc] initWithMetadata:headers
                                                                                 handler:nil]]];

src/objective-c/GRPCClient/private/GRPCRequestHeaders.h

@@ -34,9 +34,9 @@
 #import <Foundation/Foundation.h>
 #include <grpc/grpc.h>
 
-@class GRPCCall;
+#import "GRPCCall.h"
 
-@interface GRPCRequestHeaders : NSObject
+@interface GRPCRequestHeaders : NSObject<GRPCRequestHeaders>
 
 @property(nonatomic, readonly) NSUInteger count;
 @property(nonatomic, readonly) grpc_metadata *grpc_metadataArray;

src/objective-c/GRPCClient/private/GRPCWrappedCall.h

@@ -35,8 +35,7 @@
 #include <grpc/grpc.h>
 
 #import "GRPCChannel.h"
-
-@class GRPCRequestHeaders;
+#import "GRPCCall.h"
 
 @interface GRPCOperation : NSObject
 @property(nonatomic, readonly) grpc_op op;
@@ -46,7 +45,7 @@
 
 @interface GRPCOpSendMetadata : GRPCOperation
 
-- (instancetype)initWithMetadata:(GRPCRequestHeaders *)metadata
+- (instancetype)initWithMetadata:(id<GRPCRequestHeaders>)metadata
                          handler:(void(^)())handler NS_DESIGNATED_INITIALIZER;
 
 @end

src/objective-c/GRPCClient/private/GRPCWrappedCall.m

@@ -38,8 +38,6 @@
 #include <grpc/byte_buffer.h>
 #include <grpc/support/alloc.h>
 
-#import "GRPCRequestHeaders.h"
-
 #import "GRPCCompletionQueue.h"
 #import "GRPCHost.h"
 #import "NSDictionary+GRPC.h"
@@ -67,7 +65,7 @@
   return [self initWithMetadata:nil handler:nil];
 }
 
-- (instancetype)initWithMetadata:(GRPCRequestHeaders *)metadata handler:(void (^)())handler {
+- (instancetype)initWithMetadata:(id<GRPCRequestHeaders>)metadata handler:(void (^)())handler {
   if (self = [super init]) {
     _op.op = GRPC_OP_SEND_INITIAL_METADATA;
     _op.data.send_initial_metadata.count = metadata.count;