[FR] Bulldozer to listen for certain commands in comments for one-time workflows

[mdaudali]

// Similar to #95

Current state of the world

Currently, to enable one-time workflows where repositories need to merge the main branch in to a large number of PRs (e.g, to pick up a critical fix), we must update the bulldozer config to add support for update me, add the update tag, wait for the commit to be merged, remove the update tag, and revert the config change (to prevent contributors from potentially abusing the update tag to cause a storm of commits, overwhelming Github and CI). This is unnecessarily time consuming (albeit quicker than pulling repo and manually merging each one in turn).

Proposed solution

Bulldozer to listen for @bulldozer update me. Bulldozer would then update the PR where the comment was added by merging in develop.