[DO NOT MERGE] test: verify signing and DCO

[elevran]

The below is for using SSH keys. GPG keys will be similar.
Note that -S and -s are both required. The -s or --signoff simply adds a Signed-Off-By statement and the -S signing uses private-public cryptographic keys to verify the user making the commit.

Prerequisites

• Use an existing SSH key if you have it or create a new one.
  https://docs.github.com/en/authentication/connecting-to-github-with-ssh/adding-a-new-ssh-key-to-your-github-account
• Configure local git CLI to use the key (if you haven't done so git config set ... - optionally using --global)

user.signingkey=~/.ssh/<private key file>
user.email=<your email - should match the email used in the key pair>
user.name=<your full name> 
gpg.format=ssh
commit.gpgsign=true # automatically sign commits, no need for `-S` 

• Make sure to add the PUBLIC part of the key to GitHub. It MUST be added as a SIGNING key. You may also add it as an authentication key to GH (i.e., it will be added twice).

Add an empty commit to this PR and watch for the green verified mark next to it

git fetch upstream dco_check
git checkout dco_check
# -S is only needed if commit.gpgsign=true was not set in git config above. Does not hurt to add anyway...
git commit --allow-empty -S -s -m "test: verify signing and DCO"
git push upstream --set-upstream dco_check