Add command that rerolls assignment based on PR contents (`[assign.owners]`)

The initial PR assignment takes into account all the data in triagebot.toml about who reviews which part of the codebase. There's no way to re-sample that set of reviewers though; if I want to reroll, I need to do e.g. `r? compiler` and just hope that I find someone who knows the right area.

Would be good to have a command that does a re-roll taking into account the `assign.owners` table.