GitLab integration: store connection credentials

[emyller]

Context: #7000

Users connecting their GitLab instance to Flagsmith need to provide an instance URL and an access token with api scope. The integration supports personal access tokens, group access tokens, and project access tokens. The token type determines which projects are accessible.

Scope

Store GitLab credentials at the organisation level. Two fields:

• GitLab Instance URL (e.g. https://gitlab.com or https://gitlab.example.com)
• Access Token (api scope)

Note

#7121 contains a GitLabConfiguration model and CRUD viewset that can be used as a starting point. A few things to adjust:

• The model includes gitlab_project_id, project_name, and gitlab_group_path. These are not needed — the token type determines project visibility.
• webhook_secret is a user-provided field. Flagsmith will create webhooks automatically (GitLab integration: receive webhooks for automatic state sync #7165) and can generate and store secrets itself, so this doesn't need to be in the create flow.
• tagging_enabled and its lifecycle hooks belong in the labelling step (GitLab integration: label linked issues and MRs #7163).

Acceptance criteria

• GitLabConfiguration model stores instance URL and access token at the Flagsmith project level
• Organisation admins can add and remove a GitLab connection in the Project integrations tab
• Emit structlog events for product telemetry (Epic: Open structured event pipeline for the backend #7012)