[atdgen] support deriving annotations

[aryx]

Just like we can now annotate ATD type definitions with @DataClass annotations for Python, it would
be nice to be able to do the same for OCaml.
This would avoid having to write boilerplate code like: https://github.com/returntocorp/semgrep/blob/develop/semgrep-core/src/core/Output_from_core_util.ml

It would be nice if we could also define a default deriving for the whole file, to avoid having to repeats
those deriving for every definitions in the file (the same is true for Python).