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)