feat: add tls support to gitlab stub server

What does this MR do?

Example command: go run test/gitlabstub/cmd/server/main.go --key-file cert-key --cert-file cert-cert

For certificate key and certificate file see https://gitlab.com/gitlab-org/gitlab-pages/-/blob/master/internal/fixture/fixtures.go

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