Add KeyValue expression

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

tests/Function/FilterFunctionTest.php

@@ -0,0 +1,25 @@
+<?php
+declare(strict_types=1);
+
+use Arendsen\FluxQueryBuilder\Expression\KeyValue;
+use Arendsen\FluxQueryBuilder\Function\Filter;
+use PHPUnit\Framework\TestCase;
+
+final class FilterFunctionTest extends TestCase {
+
+    public function testSimpleFilter()
+    {
+        $expression = new Filter([
+            KeyValue::set('_measurement', 'test_measurement')
+                ->and('_field', 'user')
+                ->or('_field', 'field2')
+                ->and('user', 'my_username'),
+        ]);
+
+        $query = '|> filter(fn: (r) => r._measurement == "test_measurement" and r._field == "user" or ' . 
+            'r._field == "field2" and r.user == "my_username") ';
+
+        $this->assertEquals($expression->__toString(), $query);
+    }
+
+}