Describe the proposed change(s).

Note: it's possible to achieve git sign-off's today using commitBody, although it doesn't apply to config migration PRs. The purpose of this feature is to make it a "first class" option.

Configuration

Add new config option gitSignOff with possible values "enabled", "disabled" and "auto" (default). It should be configurable in repository config.

Automatic detection

When the value is "auto" then Renovate should do similarly to when semanticCommits=auto and check recent commits for evidence of use in the repo. However, unlike semanticCommits, Renovate should set gitSignOff to "enabled" if any of the recent commits include a Signed-Off-By: line in the commit messages (by any user/author).

Effect

If gitSignOff is then "enabled", Renovate should add Signed-Off-By: {{{gitAuthor}}} as the last line of the commit message body.

This should apply to all commits, including update/upgrade commits, onboarding commits, and config migration commits.

Migration of existing settings

The :gitSignOff preset should be changed from using commitBody to using gitSignOff=enabled. Additionally, if the user configures the string Signed-Off-By: {{{gitAuthor}}} in commitBody then it should also be migrated to gitSignOff=enabled and the commitBody configuration removed if that was the only content.