GitHub App tokens for previews fix rate-limit failures

Preview workflow operations in tuist-ops now use a short-lived GitHub App installation token instead of a long-lived personal token. This makes preview requests no longer share the tuistit user’s GitHub rate-limit bucket, resolving failures when publishing preview progress back to Slack. The Helm chart accepts the new github_app_id, github_app_installation_id, and github_app_private_key values.