Apply custom headers in RCTMultipartDataTask (#55607)

Summary:
Pull Request resolved: #55607

Inject custom devsupport headers into bundle download requests. This ensures that multipart bundle fetches include any headers registered via `RCTDevSupportHttpHeaders`.

Changelog:
[Internal] [Changed] -

Reviewed By: cipolleschi

Differential Revision: D93596721

fbshipit-source-id: 358b0de351677b843a70204caa101531941789b0

Author: cortinico

packages/react-native/React/Base/RCTMultipartDataTask.m

@@ -7,6 +7,8 @@
 
 #import "RCTMultipartDataTask.h"
 
+#import "RCTDevSupportHttpHeaders.h"
+
 @interface RCTMultipartDataTask () <NSURLSessionDataDelegate, NSURLSessionDataDelegate>
 
 @end
@@ -40,6 +42,7 @@ - (void)startTask
                                                    delegateQueue:nil];
   NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:_url];
   [request addValue:@"multipart/mixed" forHTTPHeaderField:@"Accept"];
+  [[RCTDevSupportHttpHeaders sharedInstance] applyHeadersToRequest:request];
   NSURLSessionDataTask *dataTask = [session dataTaskWithRequest:request];
   [dataTask resume];
   [session finishTasksAndInvalidate];