fix: Keep source/binary compatibility with 2.5.1 for addRequestHeader (#2022)

Author: johanandren

runtime/src/main/scala/akka/grpc/javadsl/AkkaGrpcClient.scala

@@ -16,7 +16,10 @@ trait AkkaGrpcClient {
   /**
    * @return The same client decorated to add the given key and value to the metadata of any request issued.
    */
-  def addRequestHeader(key: String, value: String): AkkaGrpcClient
+  def addRequestHeader(key: String, value: String): AkkaGrpcClient = {
+    // dummy implementation to not break compatibility
+    this
+  }
 
   /**
    * Initiates a shutdown in which preexisting and new calls are cancelled.

runtime/src/main/scala/akka/grpc/scaladsl/AkkaGrpcClient.scala

@@ -33,5 +33,8 @@ trait AkkaGrpcClient {
   /**
    * The same client instance decorated to add the given key and value to the metadata of any request issued.
    */
-  def addRequestHeader(key: String, value: String): AkkaGrpcClient
+  def addRequestHeader(key: String, value: String): AkkaGrpcClient = {
+    // dummy implementation to not break compatibility
+    this
+  }
 }