Add support for Gitaly GPG signing

What does this MR do?

Gitaly recently added the option to GPG sign all commits created by GitLab with gitlab-org/gitaly!4802 (merged), such as changes made via the WebIDE, merge commits, etc. This MR adds support for this option to the Gitaly chart.

Related issues

Relates gitlab-org/gitlab#19185 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
  • gitlab-org/omnibus-gitlab!6294 (merged)