cluster: Add None backend

Mostly so we have the CLUSTER_BACKEND_NONE enum value available, but
potentially useful for testing, too. The rough idea is that Zeek by
default has Cluster::backend set to CLUSTER_BACKEND_NONE, and script
packages in frameworks/cluster/backend redef it accordingly.

src/cluster/backend/CMakeLists.txt

@@ -17,3 +17,5 @@ if (ENABLE_CLUSTER_BACKEND_ZEROMQ)
 
     add_subdirectory(zeromq)
 endif ()
+
+add_subdirectory(none)

src/cluster/backend/none/CMakeLists.txt

@@ -0,0 +1,3 @@
+zeek_add_plugin(
+    Zeek Cluster_Backend_None
+    SOURCES Plugin.cc)

src/cluster/backend/none/Plugin.cc

@@ -0,0 +1,57 @@
+// See the file "COPYING" in the main distribution directory for copyright.
+
+#include "zeek/plugin/Plugin.h"
+
+#include "zeek/cluster/Backend.h"
+#include "zeek/cluster/Component.h"
+#include "zeek/cluster/Serializer.h"
+
+namespace {
+
+using namespace zeek::cluster;
+
+/**
+ * A backend that does nothing.
+ */
+class NoneBackend : public Backend {
+private:
+    bool DoInit() override { return true; };
+    void DoTerminate() override {};
+    void DoInitPostScript() override {};
+    bool DoPublishLogWrites(const zeek::logging::detail::LogWriteHeader& header, const std::string& format,
+                            zeek::byte_buffer& buf) override {
+        return true;
+    }
+    bool DoPublishEvent(const std::string& topic, const std::string& format, const zeek::byte_buffer& buf) override {
+        return true;
+    };
+    bool DoSubscribe(const std::string& topic_prefix, SubscribeCallback cb) override { return true; };
+    bool DoUnsubscribe(const std::string& topic_prefix) override { return true; };
+
+public:
+    NoneBackend(std::unique_ptr<EventSerializer> es, std::unique_ptr<LogSerializer> ls,
+                std::unique_ptr<detail::EventHandlingStrategy> ehs)
+        : Backend("None", std::move(es), std::move(ls), std::move(ehs)) {}
+
+    static std::unique_ptr<Backend> Instantiate(std::unique_ptr<EventSerializer> es, std::unique_ptr<LogSerializer> ls,
+                                                std::unique_ptr<detail::EventHandlingStrategy> ehs) {
+        return std::make_unique<NoneBackend>(std::move(es), std::move(ls), std::move(ehs));
+    }
+};
+} // namespace
+
+
+namespace zeek::plugin::Zeek_Cluster_None {
+
+class Plugin : public zeek::plugin::Plugin {
+    zeek::plugin::Configuration Configure() override {
+        AddComponent(new cluster::BackendComponent("None", NoneBackend::Instantiate));
+
+        zeek::plugin::Configuration config;
+        config.name = "Zeek::Cluster_Backend_None";
+        config.description = "Cluster backend none";
+        return config;
+    }
+} plugin;
+
+} // namespace zeek::plugin::Zeek_Cluster_None