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 gitaly!4802 (merged), such as changes made via the WebIDE, merge commits, etc. This MR adds a new gitaly['gpg_signing_key_path'] option to make the new setting accessible in Omnibus.

Related issues

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
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated -> Feat(doc): docs on Gitaly GPG signing (gitlab!96098 - merged)
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened