Add "members_count" and "parent_id" data on namespaces API

Related to https://gitlab.com/gitlab-com/customers-gitlab-com/issues/122 and https://gitlab.com/gitlab-com/customers-gitlab-com/issues/123.

This MR adds the information of members_count and parent_id on Namespace::Entity in order to being consumed by GET /namespaces endpoint. This data will be primarily used by customers.gitlab.com.

Note that using GET /namespaces instead GET /groups endpoint give us the ability of having User#namespace and Groups information with a single request.

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

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