add API to get a single artifact

Author: itscaro

lib/Gitlab/Api/Jobs.php

@@ -85,6 +85,20 @@ public function artifactsByRefName($project_id, $ref_name, $job_name)
         ))->getBody();
     }
 
+    /**
+     * @param int|string $project_id
+     * @param string $ref_name
+     * @param string $job_name
+     * @param string $artifact_path
+     * @return StreamInterface
+     */
+    public function artifactByRefName($project_id, $ref_name, $job_name, $artifact_path)
+    {
+        return $this->getAsResponse("projects/".$this->encodePath($project_id)."/jobs/artifacts/".$this->encodePath($ref_name)."/raw/".$this->encodePath($artifact_path), array(
+            'job' => $this->encodePath($job_name)
+        ))->getBody();
+    }
+
     /**
      * @param int|string $project_id
      * @param int $job_id