Add additional prompt for co-authors

Hi, as per my comment on https://github.com/commitizen/cz-cli/issues/678 I have created a [simple wrapper](https://github.com/p-m-p/cz-conventional-changelog-coauthors) around this adapter with an additional (optional) prompt for adding co-authors. If `defaultCoAuthors` config property or `CZ_CO_AUTHORS` environment variable are set then the prompt will be enabled and ask if you would like to add `Co-authored-by:` message trailers used by Github, Gitlab etc to attribute team members when pair/mob programming.

I would be happy to contribute the change/prompt here instead of having a separate adapter if this is something that would be accepted?

Many thanks

Start by reviewing the existing commitizen adapter and the linked cz-conventional-changelog-coauthors wrapper to compare how the optional prompt and configuration are handled. The work is done when the adapter supports the defaultCoAuthors configuration property and CZ_CO_AUTHORS environment variable, conditionally prompts for co-authors, and produces Co-authored-by: trailers.

Written by the indexing model from the issue text.