feat: CI auto-login

Description

This change set implements config mapping key fallbacks for the predefined CI/CD variables in case the experimental FF_GLAB_ENABLE_CI_AUTOLOGIN feature flag is enabled and the GITLAB_CI environment variable is set to true.

Even if this condition is met, the predefined CI/CD variables are only fallbacks so that users can still overwrite that behavior.

Refs &9948

An example job with the FF enabled would look like this (note the absence of glab auth login plumbing):

release:
  stage: release
  image: registry.gitlab.com/gitlab-org/cli:latest
  rules:
    - if: $CI_COMMIT_TAG
  variables:
    GLAB_ENABLE_CI_AUTOLOGIN: 'true'
  script:
    - |
      glab release create "$CI_COMMIT_TAG" \
      --name "${CI_COMMIT_TAG}" \
      --notes "Your release notes here"