Add Uploadable interface to UnitPhpSdk

Introduced a new Uploadable interface under Contracts in the UnitPhpSdk directory. This interface consists of a single method for uploading requests, which will help reduce repetitive code in individual classes/methods that require request handling. Future classes will now be able to implement this interface for consistent and simplified uploading processes.

src/Contracts/Uploadable.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace UnitPhpSdk\Contracts;
+
+use UnitPhpSdk\Http\UnitRequest;
+
+interface Uploadable
+{
+    public function upload(UnitRequest $request);
+}