feat: add plugin backend

This new plugin backend allows loading a shared object to
implement the backend storage logic. See the following for
caveats with Go plugins: https://pkg.go.dev/plugin#hdr-Warnings.

This will also need cgo enabled within the builds, which is
currently disabled for github releases. So this will require
building the gateway from source for now.

cmd/versitygw/main.go

@@ -98,6 +98,7 @@ func main() {
 		scoutfsCommand(),
 		s3Command(),
 		azureCommand(),
+		pluginCommand(),
 		adminCommand(),
 		testCommand(),
 		utilsCommand(),

cmd/versitygw/plugin.go

@@ -0,0 +1,64 @@
+// Copyright 2025 Versity Software
+// This file is licensed under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+package main
+
+import (
+	"fmt"
+
+	"github.com/urfave/cli/v2"
+	vgwplugin "github.com/versity/versitygw/backend/plugin"
+)
+
+var (
+	pluginPath   string
+	pluginConfig string
+)
+
+func pluginCommand() *cli.Command {
+	return &cli.Command{
+		Name:        "plugin",
+		Usage:       "plugin storage backend",
+		Description: `This tells the gateway to load the backend from a dynamic runtime plugin.`,
+		Action:      runPlugin,
+		Flags: []cli.Flag{
+			&cli.StringFlag{
+				Name:        "file",
+				Usage:       "path to plugin shared object file",
+				Value:       "",
+				Required:    true,
+				EnvVars:     []string{"VGW_PLUGIN_FILE"},
+				Destination: &pluginPath,
+				Aliases:     []string{"f"},
+			},
+			&cli.StringFlag{
+				Name:        "config",
+				Usage:       "configuration option for the plugin",
+				Value:       "",
+				Required:    true,
+				EnvVars:     []string{"VGW_PLUGIN_CONFIG"},
+				Destination: &pluginConfig,
+				Aliases:     []string{"c"},
+			},
+		},
+	}
+}
+
+func runPlugin(ctx *cli.Context) error {
+	be, err := vgwplugin.NewPluginBackend(pluginPath, pluginConfig)
+	if err != nil {
+		return fmt.Errorf("init plugin backend: %w", err)
+	}
+	return runGateway(ctx.Context, be)
+}