future-private: add flash close position request

src/Requests/FlashClosePositionRequestContract.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace Msr\LaravelBitunixApi\Requests;
+
+use Psr\Http\Message\ResponseInterface;
+
+interface FlashClosePositionRequestContract
+{
+    /**
+     * Flash close position by position ID
+     *
+     * @param string $positionId Position ID
+     * @return ResponseInterface
+     */
+    public function flashClosePosition(string $positionId): ResponseInterface;
+}