Support Tart Guest Agent transport for VM startup scripts (#490)

* Support Tart Guest Agent transport for VM startup scripts

* TestTartGuestAgentStartupScript: make ScriptContent more readable

* Align Tart Guest Agent's startup script execution with SSH

* Use mvdan.cc/sh/v3/syntax to validate "export" shell commands

* Use Bash instead of Zsh on Linux

* Invert logic when validating vm.StartupScript.Transport for simplicity

* Document startup_script's transport field in OpenAPI specification

* Prevent unbounded buffering of startup script output

* grpc.WaitForReady(true) → retry.DoWithData()

api/openapi.yaml

@@ -800,8 +800,16 @@ components:
           default: admin
         startup_script:
           type: object
-          description: Startup script to run after the VM boots and becomes accessible via SSH
+          description: Startup script to run after the VM boots
           properties:
+            transport:
+              type: string
+              description: |
+                Transport used to execute the startup script.
+                Omitted or empty values use SSH.
+                `tart-guest-agent` is supported only with the Tart runtime.
+              enum: [ "", ssh, tart-guest-agent ]
+              default: ssh
             script_content:
               type: string
             env: