feat: Support OAuth2 Access Token only authentication

Timo Furrerrequested to merge
oauth2-access-token-only-support into main

Description

This change set implements support for authenticating with an OAuth2 Access Token directly. That is, not using a traditional OAuth2 access flow that would include the refresh token etc, but simply be given the access token to use for authentication.

This supports integration into 3rd-party tools that "own" the OAuth2 tokens and just want to use glab to integrate with GitLab

Refs gitlab-org/developer-relations/contributor-success/ai-tooling-engineering/ai-tooling-issues#15

Related Issues

Resolves #[issue_number]

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Edited by Timo Furrer

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