[FEATURE]: GitLab Duo Provider

[mechamogeo]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Problem Currently, OpenCode does not support GitLab Duo as a provider. Many users, particularly in enterprise environments, rely on GitLab Duo for its privacy controls and existing integration with their GitLab repositories.

Proposed Solution Add gitlab as a supported provider, allowing users to connect OpenCode to GitLab Duo using their existing access tokens or oauth integrations based in opensource VS Code extension.

Reference The VS Code extension is open source and documents how to implement the code chat with Duo. This could serve as a reference for the API implementation if it's possible: https://gitlab.com/gitlab-org/gitlab-vscode-extension