Support custom headers for additional authentication tokens

Problem to Solve

Some GitLab instances are setup behind additional authentication layers (Cloudflared, Google's IAP and others). These authentication layers typically require tools to pass a token in all requests that authenticates with that service.

Proposal

The GitLab CLI should support passing additional tokens to support this auth.

Additional Details

• Cloudflared Docs for generating a token
  • cf-access-token: $TOKEN