feat: extract gitlab stub server in a separate package

What does this MR do?

Extract gitlab stub server in a separate package and allow running Pages outside the GDK.

This should people working on Pages.

Setup:

$ go run test/gitlabstub/main/main.go
2022/04/14 00:00:00 listening on http://127.0.0.1:40903

Then use

artifacts-server=http://127.0.0.1:40903/api/v4
gitlab-server=http://127.0.0.1:40903

TODO

• I added the Changelog trailer (e.g. Changelog: added) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines