upstreams: add gitlab as upstream option

[cpanato]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Add a GitLab as a new upstream option to be used to validate and check dependencies

It supports both the public GitLab and also a private (self-hosted) GitLab

The behavior is very similar to the GitHub one.
You will need to set the environment variable GITLAB_TOKEN or GITLAB_PRIVATE_TOKEN

a live run 😄

$ ./zeitgeist validate  --remote
Update available for dependency gitlabtest: 3.10.0 (current: 2.5.0)
Update available for dependency internal: 1.2.0 (current: 0.5.0)

using this sample dependencies.yaml

dependencies:
- name: gitlabtest
  version: 2.5.0
  upstream:
    flavour: gitlab
    url: harbottle/puppet-mattermost
- name: internal
  version: 0.5.0
  upstream:
    flavour: gitlab
    server: https://gitlab.eng.company.com/
    url: cpanatojr/testing-release-api

This is an initial, there more things to add, like more tests

/assign @justaugustus @saschagrunert @hasheddan

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

upstreams: add gitlab as upstream option

[Pluies]

@cpanato woohoo, that sounds great!! I'll review the code asap, looking forward to it! :)

[justaugustus]

This is great work, @cpanato!
Let's iterate on anything we need to, since it looks like you've already started working on testing this. :)

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cpanato, justaugustus

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