RFC for List Subcommand

[carolynzech]

RFC for the kani list subcommand.

I saw the comment in the template to leave the Software Design section empty, but I was looking at the raw MD file and made the mistake of thinking it referred to all sections below the comment. In other words, I thought when it said "We recommend you to leave this empty for the first version of your RFC" it meant that everything below the comment should be empty (so Rationale, Open Questions, and Future Work) and not Software Design. I realized my mistake when I was making this PR, but I figured I may as well leave it since I already wrote it. I opened a PR to update the comment.

Resolves #2573, #1612.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[carolynzech]

Thanks for the feedback @celinval! Addressed your comments.