Rerender and other bot commands with a label

[asmeurer]

The automerge label is useful. You should be able to do other bot commands like rerendering by adding a label to a PR. That would be much easier and also more discoverable than using a comment.

[h-vetinari]

This would be nice to have, but limited in utility to those who already have maintainer rights on a feedstock - regular contributors cannot set labels.