feat: integrate plugin loader into CLI flow

Complete the plugin system implementation by integrating the plugin loader
into the CLI workflow. This connects the previously added components to create
a fully functional plugin system.

Changes:
- Updated the lint method to load plugins before running the linter
- Added informative message showing how many plugins were loaded
- Ensured proper error handling and propagation

With this change, the plugin system is now complete and operational.
Users can now extend the api-linter with custom rules by:
1. Creating plugins with the AddCustomRules function
2. Building them as shared libraries (.so files)
3. Specifying them with the --rule-plugin flag when running api-linter

This completes the 3-part implementation for issue googleapis#1485, providing
a way for organizations to extend the linter with custom rules without
forking the entire repository.

Author: julioz

cmd/api-linter/cli.go

@@ -125,6 +125,15 @@ func (c *cli) lint(rules lint.RuleRegistry, configs lint.Configs) error {
 	if len(c.ProtoFiles) == 0 {
 		return fmt.Errorf("no file to lint")
 	}
+
+	// Load custom rule plugins if provided
+	if len(c.RulePluginPaths) > 0 {
+		if err := loadCustomRulePlugins(c.RulePluginPaths, rules); err != nil {
+			return fmt.Errorf("failed to load custom rule plugins: %v", err)
+		}
+		fmt.Printf("Loaded %d custom rule plugin(s)\n", len(c.RulePluginPaths))
+	}
+
 	// Read linter config and append it to the default.
 	if c.ConfigPath != "" {
 		config, err := lint.ReadConfigsFromFile(c.ConfigPath)