feat(vertex): add support for Vertex AI Gemini authentication and execution

Introduce Vertex AI Gemini integration with support for service account-based authentication, credential storage, and import functionality. Added new executor for Vertex AI requests, including execution and streaming paths, and integrated it into the core manager. Enhanced CLI with `--vertex-import` flag for importing service account keys.

cmd/server/main.go

@@ -57,6 +57,7 @@ func main() {
 	var iflowLogin bool
 	var noBrowser bool
 	var projectID string
+	var vertexImport string
 	var configPath string
 	var password string
 
@@ -69,6 +70,7 @@ func main() {
 	flag.BoolVar(&noBrowser, "no-browser", false, "Don't open browser automatically for OAuth")
 	flag.StringVar(&projectID, "project_id", "", "Project ID (Gemini only, not required)")
 	flag.StringVar(&configPath, "config", DefaultConfigPath, "Configure File Path")
+	flag.StringVar(&vertexImport, "vertex-import", "", "Import Vertex service account key JSON file")
 	flag.StringVar(&password, "password", "", "")
 
 	flag.CommandLine.Usage = func() {
@@ -417,7 +419,10 @@ func main() {
 
 	// Handle different command modes based on the provided flags.
 
-	if login {
+	if vertexImport != "" {
+		// Handle Vertex service account import
+		cmd.DoVertexImport(cfg, vertexImport)
+	} else if login {
 		// Handle Google/Gemini login
 		cmd.DoLogin(cfg, projectID, options)
 	} else if codexLogin {