Extract general QueryBuilder logic into an abstract class to make it more maintainable

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

tests/Functions/FilterFunctionTest.php

@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace Tests\Functions;
 
 use Arendsen\FluxQueryBuilder\Expression\KeyValue;
+use Arendsen\FluxQueryBuilder\Expression\KeyFilter;
 use Arendsen\FluxQueryBuilder\Functions\Filter;
 use PHPUnit\Framework\TestCase;
 
@@ -31,4 +32,17 @@ final class FilterFunctionTest extends TestCase
 
         $this->assertEquals($expression->__toString(), $query);
     }
+
+    public function testKeyFilter()
+    {
+        $expression = new Filter(KeyFilter::setEqualTo('_measurement', 'test_measurement')
+            ->andEqualTo('_field', 'user')
+            ->orEqualTo('_field', 'field2')
+            ->andEqualTo('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);
+    }
 }