future-private: add getSingleAccount request

src/Requests/GetSingleAccountRequestContract.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace Msr\LaravelBitunixApi\Requests;
+
+use Psr\Http\Message\ResponseInterface;
+
+interface GetSingleAccountRequestContract
+{
+    /**
+     * Get account details with the given margin coin
+     *
+     * @param string $marginCoin Margin coin (e.g., 'USDT')
+     * @return ResponseInterface
+     */
+    public function getSingleAccount(string $marginCoin): ResponseInterface;
+}