Automatically update protos

[yaakov-h]

Here's quick late-night hack: use GitHub Actions to automatically update our protos regularly.

(I picked Sunday morning in my time zone, I think.)

The complication here is that the repository's automatic GITHUB_TOKEN can push code and create the pull request, but that will not trigger subsequent Actions - i.e. building and testing the new code.

IMO that part is pretty important - we need to know at a glance if a protobuf update contains any breaking changes, we can't blindly merge them if they remove or rename something that we're using.

The recommendation from the various docs I've come across is to use a Personal Access Token (PAT) from either a particular developer (to whom all the PRs will be attributed to), or a new Machine Account.

According to the GitHub user terms, everyone can create at most one actual personal account plus one Machine Account, which is basically a personal account used solely for automation.

Does anyone have a machine account that we can re-use for this, or do I need to create a new one?