Introduce treefmt

[utdemir]

I noticed there were some ocaml files where ocp-indent wasn't applied. I also noticed that @dorranh was passing the Python sources from an indenter, which is a good idea.

So, I wanted to ensure that our code is indented on the CI, while being able to indent Ocaml and Python at the same time (I also added nixpkgs-fmt because I could, but it's not super important).

There's a new tool called treefmt that does that. So I added that to the CI, and set it up to use:

• ocp-indent for ocaml
• black for python
• nixpkgs-fmt for nix

I didn't enabled Black support yet because I didn't want to introduce conflicts to dorranh/clientTimeouts branch. I also didn't reformatted all our files since the diff becomes massive if I do that. Do run treefmt locally if you want to see which files change. I will apply it right before we merge this branch.

I also had to update the Nixpkgs pin to get a newer version of treefmt, but we were due for an update anyway.

And I know we haven't discussed this before, so please feel free to disagree with the PR. Alternative is that we don't introduce a new tool, add our formatters to make format, run that on CI and assert on git diff being clean.

[utdemir]

Actually, let's just have a scripts/format.sh instead of treefmt, no need to have another dependency. I'll update this PR.