Cache shared runners enabled check

This change adds caching for checking if there are any shared runners
for a given Group or Namespace.

Changelog: added

config/feature_flags/development/cache_shared_runners_enabled.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/338267
 milestone: '14.2'
 type: development
 group: group::optimize
-default_enabled: false
+default_enabled: true