Add support for getting a changelog line from a PR/Issue comment

**What would you like to be added**:
Be able to read a changelog line to be included in the changelog from the contents of a comment. This could work in a similar way as `changelog-ignore` labels, where if a `changelog-comment` label is added then chronicle would look through the comments for a trigger word/command and incorporate the user-provided changelog line.

**Why is this needed**:
This would have been great for https://github.com/anchore/syft/pull/1696 where attribution should be given to multiple users, include multiple PRs, and have an odd linking back to a single issue. This might be able to be handled by chronicle automatically, but it would be great to be able to have an override on the PR directly and not think about it.

Start by examining how Chronicle currently handles changelog-ignore labels and reads PR or issue comments. Define the trigger command, the changelog-comment label behavior, and how user-provided text should represent attribution and multiple PRs; the work is done when a labeled PR or issue reliably contributes the requested changelog line.

Written by the indexing model from the issue text.