Propagate sign param to UserCommitFiles RPC

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/150456



Merged-by: Vitali Tatarintev <vtatarintev@gitlab.com>
Approved-by: Gavin Hinfey <ghinfey@gitlab.com>
Approved-by: Vitali Tatarintev <vtatarintev@gitlab.com>
Co-authored-by: Igor Drozdov <idrozdov@gitlab.com>