Automate configuration of commit signing using SSH key

[dolmen]

https://calebhearth.com/sign-git-with-ssh
https://news.ycombinator.com/item?id=32831731
https://docs.github.com/en/authentication/managing-commit-signature-verification/about-commit-signature-verification

Add --git-global-signing and --git-local-signing key options to setup Git config either global or local the the repo.