Adding gitaly-gpg binary for gpg signing

See merge request https://gitlab.com/gitlab-org/gitaly/-/merge_requests/5906



Merged-by: John Cai <jcai@gitlab.com>
Approved-by: Quang-Minh Nguyen <qmnguyen@gitlab.com>
Approved-by: James Fargher <proglottis@gmail.com>
Reviewed-by: James Fargher <proglottis@gmail.com>
Reviewed-by: Quang-Minh Nguyen <qmnguyen@gitlab.com>