Create virtual host configuration API for GitLab Pages

Description

In order to support the architecture updates for GitLab Pages, create an API so it is no longer necessary to load the virtual host configuration from a local file.

Proposal

This work has already started in this MR, finishing up that work will complete this issue.

Links / references

Original Pages API issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/58796

Related Epic: &1316 (closed)

Status update:

• Rename API::Internal class to API::Internal::Base gitlab-foss!32617 (merged)
• Extract Workhorse <-> GitLab JWT authentication to make it reusable gitlab-foss!32623 (merged)
• Add skeleton Pages internal API gitlab-foss!32732 (merged)
• Add support for custom domains to the Pages internal API gitlab-foss!32735 (merged)
• Add support for namespaces to Pages internal API !17197 (merged)
• Migrate Pages metadata on-demand !17204 (merged)
• Background migration to migrate Pages metadata !17993 (merged)
• omnibus-gitlab: Generate and symlink gitlab_pages_shared_secret omnibus-gitlab!3705 (merged)