Added Filter aggregation

Author: Spamercz

src/Aggregation/Filter.php

@@ -0,0 +1,27 @@
+<?php declare(strict_types = 1);
+
+namespace Spameri\ElasticQuery\Aggregation;
+
+/**
+ * @see https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-filter-aggregation.html
+ */
+class Filter extends \Spameri\ElasticQuery\Filter\FilterCollection
+	implements \Spameri\ElasticQuery\Aggregation\LeafAggregationInterface
+{
+
+	public function toArray(): array
+	{
+		$array = parent::toArray();
+
+		if ($array === []) {
+			$array['must'] = [];
+		}
+
+		return [
+			'filter' => [
+				'bool' => $array,
+			],
+		];
+	}
+
+}