Deprecate unused ClientInvoker interface (#2287)

Motivation:

After simplifying offloading in #1695, `ClientInvoker` interface does
not provide any value and can be removed.

Modifications:

- Remove the remaining pkg-private use of `ClientInvoker`;
- Deprecate `ClientInvoker` interface;

Result:

`ClientInvoker` interface is deprecated and can be removed in the future
releases.

servicetalk-http-api/src/main/java/io/servicetalk/http/api/ClientInvoker.java

@@ -25,9 +25,12 @@
  * trailers, for the passed {@link StreamingHttpRequest} returns a {@link Single}.
  *
  * @param <State> The {@code state} type to use.
+ * @deprecated There is no use of this interface in our codebase, it will be removed in the future releases. If you
+ * depend on it, consider replicating a similar interface in your codebase.
  */
+@Deprecated
 @FunctionalInterface
-public interface ClientInvoker<State> {
+public interface ClientInvoker<State> { // FIXME: 0.43 - remove deprecated interface
 
     /**
      * Invokes the client.

servicetalk-http-netty/src/main/java/io/servicetalk/http/netty/AbstractStreamingHttpConnection.java

@@ -20,7 +20,6 @@
 import io.servicetalk.concurrent.api.Completable;
 import io.servicetalk.concurrent.api.Publisher;
 import io.servicetalk.concurrent.api.Single;
-import io.servicetalk.http.api.ClientInvoker;
 import io.servicetalk.http.api.FilterableStreamingHttpConnection;
 import io.servicetalk.http.api.HttpConnectionContext;
 import io.servicetalk.http.api.HttpEventKey;
@@ -61,7 +60,7 @@
 import static java.util.Objects.requireNonNull;
 
 abstract class AbstractStreamingHttpConnection<CC extends NettyConnectionContext>
-        implements FilterableStreamingHttpConnection, ClientInvoker<FlushStrategy> {
+        implements FilterableStreamingHttpConnection {
     private static final IgnoreConsumedEvent<Integer> ZERO_MAX_CONCURRENCY_EVENT = new IgnoreConsumedEvent<>(0);
 
     final CC connection;
@@ -101,9 +100,8 @@ public final <T> Publisher<? extends T> transportEventStream(final HttpEventKey<
                 failed(new IllegalArgumentException("Unknown key: " + eventKey));
     }
 
-    @Override
-    public final Single<StreamingHttpResponse> invokeClient(final Publisher<Object> flattenedRequest,
-                                                            @Nullable final FlushStrategy flushStrategy) {
+    private Single<StreamingHttpResponse> makeRequest(final Publisher<Object> flattenedRequest,
+                                                      @Nullable final FlushStrategy flushStrategy) {
         return writeAndRead(flattenedRequest, flushStrategy).liftSyncToSingle(new SpliceFlatStreamToMetaSingle<>(
                 this::newSplicedResponse));
     }
@@ -135,7 +133,7 @@ public Single<StreamingHttpResponse> request(final StreamingHttpRequest request)
                 flatRequest = flatRequest.subscribeOn(connectionContext.executionContext().executor(),
                         IoThreadFactory.IoThread::currentThreadIsIoThread);
             }
-            Single<StreamingHttpResponse> resp = invokeClient(flatRequest, determineFlushStrategyForApi(request));
+            Single<StreamingHttpResponse> resp = makeRequest(flatRequest, determineFlushStrategyForApi(request));
             if (strategy.isMetadataReceiveOffloaded()) {
                 resp = resp.publishOn(
                         connectionContext.executionContext().executor(),