Added onRequestPreparedAsync on hooks

protoc-gen-tart/main.go

@@ -238,7 +238,7 @@ Future<List<int>> doProtobufRequest(twirp.Context ctx, Uri url,
     req.bodyBytes = msgReq.writeToBuffer();
 
     // call onRequestPrepared hook for user to modify request
-    ctx = hooks.onRequestPrepared(ctx, req);
+    ctx = await hooks.onRequestPrepared(ctx, req);
 
     // send data
     final res = await httpClient.send(req);
@@ -282,7 +282,7 @@ Future<String> doJSONRequest(twirp.Context ctx, Uri url,
     req.body = json.encode(msgReq.toProto3Json());
 
     // call onRequestPrepared hook for user to modify request
-    ctx = hooks.onRequestPrepared(ctx, req);
+    ctx = await hooks.onRequestPrepared(ctx, req);
 
     // send data
     final res = await httpClient.send(req);

tart/lib/src/hooks.dart

@@ -1,12 +1,15 @@
+import 'dart:async';
+
 import 'package:http/http.dart' as http;
+
 import 'context.dart';
 import 'error.dart';
 
 /// ClientHooks contains functions that can be used on the generated client constructor.
 /// They provide callbacks for before and after the request is sent over the network.
 class ClientHooks {
   /// onRequestPrepared is after request has been prepared and immediately before request is sent
-  Context Function(Context ctx, http.Request req) onRequestPrepared;
+  Future<Context> Function(Context ctx, http.Request req) onRequestPrepared;
 
   /// onResponseReceived is only called when response has been received without error
   Function(Context ctx) onResponseReceived;
@@ -20,7 +23,7 @@ class ClientHooks {
     this.onError = defaultOnError,
   });
 
-  static Context defaultOnRequestPrepared(ctx, req) => ctx;
+  static Future<Context> defaultOnRequestPrepared(ctx, req) => ctx;
 
   static void defaultOnResponseReceived(ctx) {/* do nothing */}