make promoter info (username and email, at least) available in expressions

[Sebast1aan]

Checklist

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

Proposed Feature

When currently using the git-commit promotion step, the commits are done with the default user kargo (if no explicit author.name is set). If a promotion is done manually, it would be useful that this becomes kargo-$logged-in-user (such as kargo-admin, in case you log in with the admin account). In the promotion history you can already see who initiated the promotion, but in the git commit log itself there is no trace of it.

Motivation

This would enable to easily see who initiated a git commit from within kargo.

[krancour]

I think the most sensible thing here might be to make promoter information available within expressions, so you can dynamically populate the existing author info fields.