Lay out initial parts for the Storage framework

This includes a manager, component manager, BIF and script code, and
parts to support new storage backend plugins.

src/storage/Manager.h

@@ -0,0 +1,52 @@
+// See the file "COPYING" in the main distribution directory for copyright.
+
+#pragma once
+
+#include "zeek/plugin/ComponentManager.h"
+#include "zeek/storage/Backend.h"
+#include "zeek/storage/Component.h"
+
+namespace zeek::storage {
+
+class Manager final : public plugin::ComponentManager<Component> {
+public:
+    Manager();
+    ~Manager() = default;
+
+    /**
+     * Initialization of the manager. This is called late during Zeek's
+     * initialization after any scripts are processed.
+     */
+    void InitPostScript();
+
+    /**
+     * Opens a new storage backend.
+     *
+     * @param type The tag for the type of backend being opened.
+     * @param options A record val representing the configuration for this
+     * type of backend.
+     * @return A pair containing a pointer to a backend and a string for
+     * returning error messages if needed.
+     */
+    zeek::expected<BackendPtr, std::string> OpenBackend(const Tag& type, RecordValPtr options);
+
+    /**
+     * Closes a storage backend.
+     */
+    void CloseBackend(BackendPtr backend);
+
+    // TODO:
+    // - Hooks for storage-backed tables?
+    // - Handling aggregation from workers on a single manager?
+
+private:
+    std::vector<BackendPtr> backends;
+};
+
+} // namespace zeek::storage
+
+namespace zeek {
+
+extern storage::Manager* storage_mgr;
+
+} // namespace zeek