Add github CI

[art-w]

• Added conf-libmongoc.opam to use opam depexts for the dependency: Sadly I could not find libmongoc for all OS... Enabling ocaml-ci would help verify that the other package names are correct because I could not test them as github CI only provides ubuntu/mac/windows (but AFAIK the repo needs to be public first for ocaml-ci).
• The test omont.t assumes that an instance of MongoDB is running and that the mongosh binary is installed... This setup was easy to reproduce for ubuntu, but for macos I only test if it builds. This will cause some issues when publishing with the opam CI since mongo will not be runnable easily (and ocaml-ci will definitively not be happy)
• Finally the documentation is automatically available at https://tarides.github.io/mongoc-ocaml/mongoc/Mongoc/index.html :)

[cuihtlauac]

but AFAIK the repo needs to be public first for ocaml-ci

The repo is already public, @samoht has changed the settings earlier this week.

The test omont.t assumes that an instance of MongoDB is running and that the mongosh binary is installed... This setup was easy to reproduce for ubuntu, but for macos I only test if it builds

Overall I'm unsure of what to test, under which systems and hypothesis regarding a binding.

I wrote this cram test to enable comprehensive changes without worrying too much. I'm unsure if it's the right approach in the long run.

[cuihtlauac]

Thanks @art-w, that looks good to me

[art-w]

Oh right it's public thanks! I enabled ocaml-ci on the repo but we should expect some issues with the test and the depexts