Add classes and tests from common project

src/Configuration.php

@@ -0,0 +1,32 @@
+<?php declare(strict_types=1);
+
+namespace BitBadger\PDODocument;
+
+/**
+ * Common configuration for the document library
+ */
+class Configuration
+{
+    /** @var string The name of the ID field used in the database (will be treated as the primary key) */
+    private static string $_idField = 'id';
+
+    /**
+     * Configure the ID field used by the library
+     *
+     * @param string $name The name of the ID field within each document
+     */
+    public static function useIdField(string $name): void
+    {
+        self::$_idField = $name;
+    }
+
+    /**
+     * Retrieve the ID field for documents within this library
+     *
+     * @return string The configured ID field
+     */
+    public static function idField(): string
+    {
+        return self::$_idField;
+    }
+}