Create a bot to trigger benchmarks using a comment on PRs

Right now our workflow for triggering benchmarks is to add a label on a PR that we want to run benchmarks on, then push a change to run the benchmark. This is not as ideal since:

• If you don't have any pending changes, you need to push an empty diff to trigger the benchmark.
• Every new change triggers a new benchmark run, which wastes CI time.

We should look into using a bot that picks up comments on PRs to trigger benchmarks.