fix(kafka_producer): add back `is_buffer_supported` callback

Fixes https://emqx.atlassian.net/browse/EMQX-9366

This callback was accidentally removed while adding another feature,
which made the buffer workers to be used for this bridge while they
shouldn’t be.

lib-ee/emqx_ee_bridge/src/emqx_ee_bridge.app.src

@@ -1,6 +1,6 @@
 {application, emqx_ee_bridge, [
     {description, "EMQX Enterprise data bridges"},
-    {vsn, "0.1.7"},
+    {vsn, "0.1.8"},
     {registered, [emqx_ee_bridge_kafka_consumer_sup]},
     {applications, [
         kernel,

lib-ee/emqx_ee_bridge/src/kafka/emqx_bridge_impl_kafka_producer.erl

@@ -7,6 +7,7 @@
 
 %% callbacks of behaviour emqx_resource
 -export([
+    is_buffer_supported/0,
     callback_mode/0,
     on_start/2,
     on_stop/2,
@@ -26,6 +27,8 @@
 %% to hocon; keeping this as just `kafka' for backwards compatibility.
 -define(BRIDGE_TYPE, kafka).
 
+is_buffer_supported() -> true.
+
 callback_mode() -> async_if_possible.
 
 %% @doc Config schema is defined in emqx_ee_bridge_kafka.