feat: add rule-plugin flag to CLI

Add a new flag to the api-linter CLI that allows specifying custom rule plugins.
This is the first step toward supporting a plugin ecosystem for custom rules.

Changes:
- Added RulePluginPaths field to the cli struct to store plugin file paths
- Added --rule-plugin flag that can be used multiple times for multiple plugins
- Updated tests to verify the flag works correctly

The flag follows the pattern of other path-related flags in the CLI:
- Uses StringArrayVar to support multiple values
- Consistent naming with other flags (kebab-case)
- Clear documentation in the help text

This is part of the implementation for issue googleapis#1485, enabling organization-specific
custom rules through plugins rather than requiring users to fork the repository.

References:
- Issue discussion: googleapis#1485
- Go plugin docs: https://pkg.go.dev/plugin

Author: julioz

cmd/api-linter/cli.go

@@ -46,6 +46,7 @@ type cli struct {
 	ListRulesFlag             bool
 	DebugFlag                 bool
 	IgnoreCommentDisablesFlag bool
+	RulePluginPaths           []string
 }
 
 // ExitForLintFailure indicates that a problem was found during linting.
@@ -67,6 +68,7 @@ func newCli(args []string) *cli {
 	var listRulesFlag bool
 	var debugFlag bool
 	var ignoreCommentDisablesFlag bool
+	var rulePluginFlag []string
 
 	// Register flag variables.
 	fs := pflag.NewFlagSet("api-linter", pflag.ExitOnError)
@@ -82,6 +84,7 @@ func newCli(args []string) *cli {
 	fs.BoolVar(&listRulesFlag, "list-rules", false, "Print the rules and exit.  Honors the output-format flag.")
 	fs.BoolVar(&debugFlag, "debug", false, "Run in debug mode. Panics will print stack.")
 	fs.BoolVar(&ignoreCommentDisablesFlag, "ignore-comment-disables", false, "If set to true, disable comments will be ignored.\nThis is helpful when strict enforcement of AIPs are necessary and\nproto definitions should not be able to disable checks.")
+	fs.StringArrayVar(&rulePluginFlag, "rule-plugin", nil, "The path to a custom rule plugin (.so file).\nMay be specified multiple times.")
 
 	// Parse flags.
 	err := fs.Parse(args)
@@ -103,6 +106,7 @@ func newCli(args []string) *cli {
 		ListRulesFlag:             listRulesFlag,
 		DebugFlag:                 debugFlag,
 		IgnoreCommentDisablesFlag: ignoreCommentDisablesFlag,
+		RulePluginPaths:           rulePluginFlag,
 	}
 }
 

cmd/api-linter/cli_test.go

@@ -44,6 +44,17 @@ func TestNewCli(t *testing.T) {
 				ProtoFiles:              []string{},
 			},
 		},
+		{
+			name: "RulePlugins",
+			inputArgs: []string{
+				"--rule-plugin=plugin1.so",
+				"--rule-plugin=plugin2.so",
+			},
+			wantCli: &cli{
+				RulePluginPaths: []string{"plugin1.so", "plugin2.so"},
+				ProtoFiles:      []string{},
+			},
+		},
 	}
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {