feat: Introduce add-llm-coauthor script

* Added an interactive script to add LLM `Co-authored-by` trailers to git
commits.
* Utilized fzf for selecting multiple commits and LLM assistants.
* Supported modifying both the HEAD commit via amend and older commits via
rebase.
* Prevented the addition of duplicate LLM co-author trailers.
* Automatically determined a relevant commit range, defaulting to commits not
on origin/main.

Co-authored-by: Cursor <cursor@users.noreply.github.com>
Signed-off-by: Chmouel Boudjnah <chmouel@redhat.com>

Author: chmouel