Deprecate HttpExecutionStrategy.invokeService (#1720)

Motivation:
The `invokeService` method is to be obsoleted by the upcoming
OffloadingFilter
Modifications:
`invokeService()` is deprecated
Result:
Warning provided of future method removal.

Author: bondolo

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

@@ -54,8 +54,12 @@ <FS> Single<StreamingHttpResponse> invokeClient(Executor fallback, Publisher<Obj
      * {@link Publisher} containing all data constituting an HTTP response.
      * @param errorHandler In case there is an error before calling the passed {@code service}, this {@link BiFunction}
      * will be called to generate an error response.
-     * @return A flattened {@link Publisher} containing all data constituting an HTTP response..
+     * @return A flattened {@link Publisher} containing all data constituting an HTTP response.
+     *
+     * @deprecated Will be removed as part of improved offloading strategy. If you need this API please file an issue
+     * explaining the use case.
      */
+    @Deprecated
     Publisher<Object> invokeService(Executor fallback, StreamingHttpRequest request,
                                     Function<StreamingHttpRequest, Publisher<Object>> service,
                                     BiFunction<Throwable, Executor, Publisher<Object>> errorHandler);