feat: add support for plugin index

.github/workflows/semantic-pr.yaml

@@ -44,6 +44,7 @@ jobs:
             k8s
             aws
             vm
+            plugin
 
             alpine
             wolfi

cmd/trivy/main.go

@@ -28,10 +28,7 @@ func main() {
 func run() error {
 	// Trivy behaves as the specified plugin.
 	if runAsPlugin := os.Getenv("TRIVY_RUN_AS_PLUGIN"); runAsPlugin != "" {
-		if !plugin.IsPredefined(runAsPlugin) {
-			return xerrors.Errorf("unknown plugin: %s", runAsPlugin)
-		}
-		if err := plugin.RunWithURL(context.Background(), runAsPlugin, plugin.RunOptions{Args: os.Args[1:]}); err != nil {
+		if err := plugin.RunWithURL(context.Background(), runAsPlugin, plugin.Options{Args: os.Args[1:]}); err != nil {
 			return xerrors.Errorf("plugin error: %w", err)
 		}
 		return nil

docs/community/contribute/pr.md

@@ -114,6 +114,7 @@ mode:
 - server
 - aws
 - vm
+- plugin
 
 os:
 

docs/docs/configuration/reporting.md

@@ -399,7 +399,7 @@ $ trivy <target> [--format <format>] --output plugin=<plugin_name> [--output-plu
 ```
 
 This is useful for cases where you want to convert the output into a custom format, or when you want to send the output somewhere.
-For more details, please check [here](../advanced/plugins.md#output-plugins).
+For more details, please check [here](../plugin/plugins.md#output-plugins).
 
 ## Converting
 To generate multiple reports, you can generate the JSON report first and convert it to other formats with the `convert` subcommand.