Add support for gitlab @@role codeowner syntax

[sbreakey]

GitLab supports the @@role syntax in its CODEOWNERS specification (e.g. @@developer, @@maintainer, @@owner), which lets you assign a code owner by project role rather than by a specific user or group. See the GitLab docs.

Currently, with "assigneesFromCodeOwners": true, when Renovate reads these entries out of CODEOWNERS and tries to resolve reviewers/assignees, expandGroupMembers doesn't recognise the @@role syntax. It strips the leading @ and passes the remainder (e.g. @developer) to the GitLab groups API as if it were a group name. No such group exists, so the request returns a 404 and the intended reviewers are never assigned.

I have implemented AI slopped (but it passes my personal review 😄 ) a fix together if it would be of interest to the project. I have not run it against a real repo, only relied on unit tests. I'm happy to give that a bash if it's a requirement.

The fix simply detects @@role handles in expandGroupMembers before the group-expansion path. When a handle maps to a known GitLab role it resolves it to the project's members at that access level (via the project members API, filtered client side by access_level as the API does not support such a filter) and expands to their usernames. This mirrors how group handles are expanded to their members. Handles that aren't recognised roles continue through the existing group/user logic unchanged.

[RahulGautamSingh]

PR welcome