Support Regex in Promotion Policies

[BenHesketh21]

Checklist

• I've searched the issue queue to verify this is not a duplicate feature request.
• I've pasted the output of kargo version, if applicable.
• I've pasted logs, if applicable.

Proposed Feature

It would be nice if promotion policies could accept regex for stages so that you can enable auto promotion for a group of stages with a similar name.

Motivation

We are trying to use Kargo for PR Preview environments and wanting to use auto promotion for those. But with the way promotion policies work. I would have to add a new policy every time a PR is created and remove it every time the PR was closed. I could put each PR in it's own project but that seems like it would quickly bloat the projects list page.

Suggested Implementation

I would just make the stage section of promotion policies accept regex, or have a similar option as credentials do and have a stageIsRegex bool option. And apply the policy to all stages in that project that fit the description.