OCaml bot for GitHub interactions

[avsm]

There have been a few requests in recent years for a more "active" bot for various repository actions to be triggered via a GitHub comment. The current requests are:

• for opam-repository maintainers to have more control over whether certain expensive tests are run, for example reverse dependencies on popular packages like dune that trigger a lot of builds.
• for benchmarking infrastructure to trigger off tests. This is being discussed in Using coqbot to run OCaml benchmarks coq/bot#288
• for package releases to be generated and tested ahead of a submission to opam-repository.

There are a couple of workflow mechanisms we can use for this, possibly in combination:

• the Coq project has https://github.com/coq/bot
• the Ocurrent project has reactive webhooks from GitHub https://github.com/ocurrent/overview

We already have several GitHub bot accounts that are suitable for use as a host for this: @bactrian and @vicuna.

This is a tracking issue for the deployment and maintainence of such a bot once the prototypes have been completed.