Add KeyValue expression

Signed-off-by: davidarendsen <davidarendsen@hey.com>

src/Expression/KeyValue.php

@@ -0,0 +1,39 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Expression;
+
+class KeyValue extends Base {
+
+    /**
+     * @var array $expressions
+     */
+    private $expressions;
+
+    public function __construct(string $key, string $value)
+    {
+        $this->expressions[] = 'r.' . $key . ' == "' . $value . '"';
+    }
+
+    public static function set(string $key, string $value): KeyValue
+    {
+        return new self($key, $value);
+    }
+
+    public function and(string $key, string $value): KeyValue
+    {
+        $this->expressions[] = 'and r.' . $key . ' == "' . $value . '"';
+        return $this;
+    }
+
+    public function or(string $key, string $value): KeyValue
+    {
+        $this->expressions[] = 'or r.' . $key . ' == "' . $value . '"';
+        return $this;
+    }
+
+    public function __toString()
+    {
+        return implode(' ', $this->expressions);
+    }
+
+}