Update handle_tls to support latest tls version

[metame]

I haven't tested this yet, just that it builds on OCaml 5.2 and the tests pass (i.e. dune test). I based the changes off of what was changed in the ocaml-tls repo for the eio subpackage as referenced at the top of ssl.ml, that diff is here. Note that the diff for ocaml-tls/eio is not very clean due to another change they were making as well, but I believe I brought over the necessary changes.

This is mostly to support the latest tls that's used in the package set for OCaml 5.2 as used in the nix-ocaml overlay. We could alternatively pin back to the 0.17.3 version manually in the flake if you prefer.