Allow to enable / disable Auto SSL (Letsencrypt) support via API

GitLab introduced in %12.1 the Auto SSL feature for GitLab Pages, using Letsencrypt. This can be enabled via a flag in project settings page on our Web UI. We should expose this to our APIs.

https://docs.gitlab.com/ee/api/pages_domains.html

1. Listing domains or specific domain should include : auto_ssl_enabled.
2. It should be possible to update auto_ssl_enabled field on existing domains or create new ones setting this value as well.