feat(pi-ssh): support SSH agent authentication

pi-ssh/src/ssh2-shim.d.ts

@@ -8,6 +8,7 @@ declare module "ssh2" {
     username: string;
     password?: string;
     privateKey?: Buffer | string;
+    agent?: string;
     passphrase?: string;
     tryKeyboard?: boolean;
     readyTimeout?: number;