Fix binary compatibility.

Recreated ClientState as wrapper around GrpcChannel to preserve
compatibility with generated clients from previous versions.

runtime/src/main/scala/akka/grpc/internal/ClientState.scala

@@ -0,0 +1,50 @@
+/*
+ * Copyright (C) 2018-2021 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package akka.grpc.internal
+
+import java.util.concurrent.CompletionStage
+
+import scala.concurrent.Future
+
+import akka.Done
+import akka.actor.ClassicActorSystemProvider
+import akka.annotation.{InternalApi, InternalStableApi}
+import akka.event.LoggingAdapter
+import akka.grpc.{GrpcChannel, GrpcClientSettings}
+
+/**
+ * INTERNAL API
+ *
+ * Deprecated: This class wraps a GrpcChannel for compatibility with clients generated by previous versions.
+ */
+@deprecated("Kept for binary compatibility between generated code and runtime", "akka-grpc 2.1.4")
+@InternalApi
+final class ClientState(channel: GrpcChannel)(
+    implicit sys: ClassicActorSystemProvider) {
+
+  @InternalStableApi
+  val internalChannel: InternalChannel =
+    channel.internalChannel
+
+  @InternalStableApi
+  def this(settings: GrpcClientSettings, log: LoggingAdapter)(implicit sys: ClassicActorSystemProvider) =
+    this(GrpcChannel(settings))
+
+  def closedCS(): CompletionStage[Done] = channel.closedCS()
+  def closeCS(): CompletionStage[Done] = channel.closeCS()
+
+  def closed(): Future[Done] = channel.closed()
+
+  def close(): Future[Done] = channel.close()
+}
+
+/**
+ * INTERNAL API
+ * Used from generated code so can't be private.
+ *
+ * Thrown if a withChannel call is called after closing the internal channel
+ */
+@InternalApi
+final class ClientClosedException() extends RuntimeException("withChannel called after close()")