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.cc

@@ -0,0 +1,58 @@
+// See the file "COPYING" in the main distribution directory for copyright.
+
+#include "zeek/storage/Manager.h"
+
+#include "zeek/Desc.h"
+
+namespace zeek::storage {
+
+Manager::Manager() : plugin::ComponentManager<storage::Component>("Storage", "Backend") {}
+
+void Manager::InitPostScript() { detail::backend_opaque = make_intrusive<OpaqueType>("Storage::Backend"); }
+
+zeek::expected<BackendPtr, std::string> Manager::OpenBackend(const Tag& type, RecordValPtr options) {
+    Component* c = Lookup(type);
+    if ( ! c ) {
+        return zeek::unexpected<std::string>(
+            util::fmt("Request to open unknown backend (%d:%d)", type.Type(), type.Subtype()));
+    }
+
+    if ( ! c->Factory() ) {
+        return zeek::unexpected<std::string>(
+            util::fmt("Factory invalid for backend %s", GetComponentName(type).c_str()));
+    }
+
+    ODesc d;
+    type.AsVal()->Describe(&d);
+
+    BackendPtr bp = c->Factory()(d.Description());
+
+    if ( ! bp ) {
+        return zeek::unexpected<std::string>(
+            util::fmt("Failed to instantiate backend %s", GetComponentName(type).c_str()));
+    }
+
+    if ( auto res = bp->Open(std::move(options)); res.has_value() ) {
+        return zeek::unexpected<std::string>(
+            util::fmt("Failed to open backend %s: %s", GetComponentName(type).c_str(), res.value().c_str()));
+    }
+
+    // TODO: post Storage::backend_opened event
+
+    backends.push_back(bp);
+
+    return bp;
+}
+
+void Manager::CloseBackend(BackendPtr backend) {
+    auto it = std::find(backends.begin(), backends.end(), backend);
+    if ( it == backends.end() )
+        return;
+
+    backends.erase(it);
+    backend->Close();
+
+    // TODO: post Storage::backend_lost event
+}
+
+} // namespace zeek::storage