GitLab integration: link issues and MRs to feature flags

[emyller]

Context: #7000

After connecting their GitLab instance (#7159), users can link GitLab issues and merge requests to their feature flags.

Scope

• Proxy endpoints to browse GitLab projects, issues, and merge requests using the stored connection credentials.
• Store the link between a feature flag and a GitLab issue or MR.

Important

Unlike GitHub, where users pre-select repositories during the App installation flow, GitLab tokens grant access to all projects within their scope. A project selection UI would expose someone else's token scope to any Flagsmith user on the integrations page. The GitLab integration exposes all accessible projects by design.

Note

#7120 and #7121 contain reference code. A few things to consider:

• GitLabResource in types.py represents an issue or MR — name is vague.
• Other client functions in feat(gitlab): add GitLab API v4 client with typed responses #7120 belong in later issues:
  • post_comment_to_gitlab → GitLab integration: post comments on linked issues and MRs #7162
  • create_flagsmith_flag_label, label_gitlab_resource → GitLab integration: label linked issues and MRs #7163
  • Webhook handling → GitLab integration: receive webhooks for automatic state sync #7165

Acceptance criteria

• In a feature flag's Links tab, users can select a GitLab project, choose between issue or merge request, and search for resources to link
• Linked GitLab resources appear in the Links tab with type, name, status, and a link to GitLab
• Emit structlog events for product telemetry (Epic: Open structured event pipeline for the backend #7012)