Add KeyValue expression

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

tests/Function/FromFunctionTest.php

@@ -0,0 +1,38 @@
+<?php
+declare(strict_types=1);
+
+use Arendsen\FluxQueryBuilder\Function\From;
+use PHPUnit\Framework\TestCase;
+
+final class FromFunctionTest extends TestCase {
+
+    /**
+     * @dataProvider somethingProvider
+     */
+    public function testSomething($settings, $query)
+    {
+        $expression = new From($settings);
+
+        $this->assertEquals($expression->__toString(), $query);
+    }
+
+    public function somethingProvider(): array
+    {
+        return [
+            'from bucket' => [
+                [
+                    'bucket' => 'test-bucket',
+                ],
+                'from(bucket: "test-bucket") '
+            ],
+            'from bucket and host' => [
+                [
+                    'bucket' => 'test-bucket',
+                    'host' => 'test',
+                ],
+                'from(bucket: "test-bucket", host: "test") '
+            ],
+        ];
+    }
+
+}