Add the Config class

Add a Config class for storing configuration values such as the
Glassboxx username, password and vendor ID.

tests/Glassboxx/ConfigTest.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace Opdavies\Glassboxx\Tests\Glassboxx;
+
+use Opdavies\Glassboxx\Config;
+use PHPUnit\Framework\TestCase;
+
+class ConfigTest extends TestCase
+{
+    public function testThatGettersReturnExpectedValues(): void
+    {
+        $config = new Config(
+            12345,
+            'opdavies',
+            'secret'
+        );
+
+        $this->assertSame(12345, $config->getVendorId());
+        $this->assertSame('opdavies', $config->getUsername());
+        $this->assertSame('secret', $config->getPassword());
+    }
+}