Implement Signed-off-by functionality for commits

Currently, there is no straightforward way to add a Signed-off-by line to commits using the standard Sapling commit workflow (sl commit). This line is important for confirming that the committer has the right to submit the work under the project's license (e.g., following the Developer Certificate of Origin).

Proposed Solution:

Add a new flag to sl commit (e.g., --signoff or -s) which, when present, appends the line Signed-off-by: <Committer Name> <Committer Email> to the commit message.

Example Use Case:

# Existing flow
sl commit -m "Fix: Corrected typo in README"

# New flow
sl commit -m "Fix: Corrected typo in README" --signoff
# The resulting commit message will contain:
# Fix: Corrected typo in README
#
# Signed-off-by: Jane Doe <[email protected]>