Implement Cluster Environments polling

What does this MR do?

This MR adds the ability to poll Cluster Environments

Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/13392

Screenshots

Loading	Found	Not found
deploy-loading	deploy-found	deploy-not-found

TODO

• Frontend polling
• Backend polling support (etag)
• Backend Specs
• Changelog
• Squash commits logically