Add Push Rule for DCO Signoff

What does this MR do and why?

Adds a Push rule to validate commits for a DCO Signoff (Signed-off-by: trailer)

Functionality and implementation is similar to the 'reject unsigned commits' push rule, except for here we just check for the trailer in the commit message.

Resolves #368894 (closed)

Builds on the Migration introduced in !97938 (merged)

Screenshots or screen recordings

Admin push rule

Project push rule

Command line:

No Signoff:

╰─$ git --no-pager log -n1
commit 72262fef62743235069a2bc48801dd1ea51e858a (HEAD -> testing)
Author: Raimund Hook <raimund@example.com>
Date:   Fri Sep 16 14:07:28 2022 +0100

    Add regular commit

╰─$ git push -u origin testing -v
Pushing to ssh://gdk.test:2222/flightjs/Flight.git
Enumerating objects: 5, done.
Counting objects: 100% (5/5), done.
Delta compression using up to 10 threads
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 294 bytes | 294.00 KiB/s, done.
Total 3 (delta 2), reused 0 (delta 0), pack-reused 0
remote: GitLab: Commit must contain DCO Sign-off
To ssh://gdk.test:2222/flightjs/Flight.git
 ! [remote rejected] testing -> testing (pre-receive hook declined)
error: failed to push some refs to 'ssh://gdk.test:2222/flightjs/Flight.git'

With Signoff:

╰─$ git commit --amend -s                                                                                           1 ↵
[testing 2d0f53e] Add regular commit
 Date: Fri Sep 16 14:07:28 2022 +0100
 1 file changed, 1 insertion(+)
╰─$ git --no-pager log -n1       
commit 2d0f53ee7ceca49005463e309da557c05424ec27 (HEAD -> testing)
Author: Raimund Hook <raimund@example.com>
Date:   Fri Sep 16 14:07:28 2022 +0100

    Add regular commit
    
    Signed-off-by: Raimund Hook <raimund@example.com>
╰─$ git push -u origin testing   
Enumerating objects: 5, done.
Counting objects: 100% (5/5), done.
Delta compression using up to 10 threads
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 316 bytes | 316.00 KiB/s, done.
Total 3 (delta 2), reused 0 (delta 0), pack-reused 0
remote: 
remote: To create a merge request for testing, visit:
remote:   http://gdk.test:3000/flightjs/Flight/-/merge_requests/new?merge_request%5Bsource_branch%5D=testing
remote: 
To ssh://gdk.test:2222/flightjs/Flight.git
 * [new branch]      testing -> testing
branch 'testing' set up to track 'origin/testing'.

How to set up and validate locally

1. Enable rule on a project
2. Commit a change that does not contain Signed-off-by: User Name <user@example.com> as a commit trailer
3. Push the commit
4. Observe that the commit is rejected
5. Sign the commit (git commit -s --amend)
6. Push the commit
7. Observe that the commit is successful

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.