Add support for a zip file storage as a backend in gitlab-pages

Description

In order to prepare for the support of object storage as a backend for GitLab Pages, the first step will be to support serving a pages site from a zip file on the existing file storage backend.

Proposal

This work has already started in this MR which can be used as a starting point for this work.

Update 2020-07-03:

This PoC MR gitlab-pages!299 (closed) uses a revised version of the package which allows serving from a zip archive.

We could take gitlab-pages!299 (closed) and split the work into:

1. Add a package zip to Pages that can open and read archives from disk or from URLs (already done in gitlab-pages!299 (closed)) with more tests.
2. Add a package zipserving that implements type Serving interface so it can be hooked into Pages easily.

Using the response from the API depends on #225840 (closed) so it would be out of scope for this issue.

Links / references

Related Epic: &1316 (closed)