Add GitLab client constructor

[lukes]

Hi!

I've added a GitLab client constructor. Unfortunately I haven't been able to have bundler resolve the gem dependencies, so wasn't able to run the specs. Please could you check out this branch and make sure the specs run for me!

You can test the client using an unauthenticated request:

GITLAB_GQL = GQLi::GitLab.create

ProjectQuery = GQLi::DSL.query {
  project(fullPath: "gitlab-org/gitlab") {
    name
  }
}

GITLAB_GQL.execute(ProjectQuery)

[lukes]

Hi, @dlitvakb! Could you please review this PR? Thank you!

[lukes]

Hi, @cachrisman @asaaki! Are you able to review this PR? Thank you!