use a static map for additional headers

packages/stream_chat/lib/src/core/http/interceptor/additional_headers_interceptor.dart

@@ -0,0 +1,17 @@
+import 'package:dio/dio.dart';
+import 'package:stream_chat/stream_chat.dart';
+
+/// Interceptor that sets additional headers for all requests.
+class AdditionalHeadersInterceptor extends Interceptor {
+  @override
+  Future<void> onRequest(
+    RequestOptions options,
+    RequestInterceptorHandler handler,
+  ) async {
+    options.headers = {
+      ...options.headers,
+      ...StreamChatClient.additionalHeaders,
+    };
+    return handler.next(options);
+  }
+}

packages/stream_chat/lib/src/core/http/interceptor/user_agent_interceptor.dart

@@ -1,20 +0,0 @@
-import 'package:dio/dio.dart';
-import 'package:stream_chat/src/core/platform_detector/platform_detector.dart';
-import 'package:stream_chat/version.dart';
-
-/// User agent interceptor that sets the user agent header
-class UserAgentInterceptor extends Interceptor {
-  final _defaultUserAgent = 'stream-chat-dart-client-'
-      '${CurrentPlatform.name}-'
-      '${PACKAGE_VERSION.split('+')[0]}';
-
-  @override
-  Future<void> onRequest(
-    RequestOptions options,
-    RequestInterceptorHandler handler,
-  ) async {
-    options.headers['X-Stream-Client'] =
-        '$_defaultUserAgent-${usedPackage.name}';
-    return handler.next(options);
-  }
-}

packages/stream_chat/lib/src/core/http/stream_http_client.dart

@@ -8,7 +8,7 @@ import 'package:stream_chat/src/core/http/connection_id_manager.dart';
 import 'package:stream_chat/src/core/http/interceptor/auth_interceptor.dart';
 import 'package:stream_chat/src/core/http/interceptor/connection_id_interceptor.dart';
 import 'package:stream_chat/src/core/http/interceptor/logging_interceptor.dart';
-import 'package:stream_chat/src/core/http/interceptor/user_agent_interceptor.dart';
+import 'package:stream_chat/src/core/http/interceptor/additional_headers_interceptor.dart';
 import 'package:stream_chat/src/core/http/stream_chat_dio_error.dart';
 import 'package:stream_chat/src/core/http/token_manager.dart';
 import 'package:stream_chat/src/location.dart';
@@ -42,7 +42,7 @@ class StreamHttpClient {
         ..._options.headers,
       }
       ..interceptors.addAll([
-        UserAgentInterceptor(),
+        AdditionalHeadersInterceptor(),
         if (tokenManager != null) AuthInterceptor(this, tokenManager),
         if (connectionIdManager != null)
           ConnectionIdInterceptor(connectionIdManager),