Commit with signed off

### Is your feature request related to a problem? Please describe.
When I want to commit with `signed off`, I don't see any possible way to do this on lazygit.

### Describe the solution you'd like
Can we add another shortcut next commit for Commit with signed off?

### Describe alternatives you've considered
Use raw terminal for commit.

### Additional context
Add any other context or screenshots about the feature request here.

Start by locating lazygit's existing commit action and its keyboard shortcut definitions, then inspect how commit options are passed to Git. The custom-command examples linked in the issue may clarify the expected signed-off behavior. Done means a distinct shortcut creates a commit with Git's signed-off metadata while preserving existing commit behavior.

Written by the indexing model from the issue text.