[11.12] Add support for filter in Projects::variable (#767)

* fix: add filter option to project variable method

This commit implements the option to add a filter[environment_scope] 
to a variable request, when multiple variables
with the same name exist, but have different
environment scopes.

See documentation for more information
https://docs.gitlab.com/ee/api/project_level_variables.html#get-a-single-variable

* fix: fix indenting and remove extra resolver

Author: fului

src/Api/Projects.php

@@ -1214,12 +1214,17 @@ public function variables($project_id, array $parameters = [])
     /**
      * @param int|string $project_id
      * @param string     $key
+     * @param array      $parameters
      *
      * @return mixed
      */
-    public function variable($project_id, string $key)
+    public function variable($project_id, string $key, array $parameters = [])
     {
-        return $this->get($this->getProjectPath($project_id, 'variables/'.self::encodePath($key)));
+        $resolver = $this->createOptionsResolver();
+        $resolver->setDefined('filter')
+            ->setAllowedTypes('filter', 'array');
+
+        return $this->get($this->getProjectPath($project_id, 'variables/'.self::encodePath($key)), $resolver->resolve($parameters));
     }
 
     /**