Add Eio collector

[shonfeder]

Closes #51

The primary addition here a collector using a Cohttp_eio client to collect signals in programs using Eio. Additional changes include:

• Some adaptations to the testing harness from Factor batching logic out of the cohttp-lwt client #97, accounting for Cohttp_eios different preferences round networking (defaulting to IPv6).
• An Eio based emitter executable that is isomorphic to the Lwt-based emit1-cottp.ml.
• An instantiation of the same signal gathering test suite used for the Lwt collector using the Eio emitter. This lets us confirm that the exact same tests which work to validate the Lwt based collector also check out for the Eio based collector.

---

Todo

• Rebase on main once Add integration tests for collectors #96 is merged
• Fix CI test failure due to needing to accommodate different sets of packages being run on different ocaml versions in the build matrix.
• Fix hack in Http client configuration, so we can support https

[shonfeder]

This should be ready for review when you have availability!

[shonfeder]

I don't think we want to fail fast any more, since we test different packages in different parts of the build matrix.

[c-cube]

That's a lot! But I think it looks solid. I only have minor qualms, then I'll merge :)

[shonfeder]

That's a lot!

Yes, I'm sorry I wasn't able to break this into smaller bits given my time frame. Tho it was smaller than the first draft at least 😅

Thank you for the review and for catching the issues you noticed!

[c-cube]

seems like the package is still dependent on eio_main. Should it be just a test-dep?

[shonfeder]

Fixed now I think!
🤦

[c-cube]

Thank you! :)