allow reading gzip compressed schemas

[ivanov]

thanks again for Exonerate! I mentioned the issue of working with compressed (gzipped) schemas at CodeBEAM America, and Isaac suggested I open an issue.

I have a bunch of schemas that are wordy but compress nicely (5-6x smaller). I currently end up generating the validators like this (code and names have been changed and simplified to protect the innocent).

defmodule CompressedReaderHelper do
  defmacro __using__(_env) do
    res = File.stream!(Path.join(__DIR__, "schema.json.gz"), [:read, :compressed]) |> Enum.into("")
    quote do
      require Exonerate
      Exonerate.function_from_string( :def, :function_name, unquote(res))
    end
  end
end

(I actually wrote it this way so I could use CompressedReaderHelper and pass some extra parameters to change the schema file path for different modules, but that's not pertinent to the compression).

I think it would be pretty straightforward to conditionally pass the :compressed option and replace File.read! with File.stream! in function_from_file https://github.com/E-xyza/Exonerate/blob/1a639563a64ee1bf2b3ad4417871c3d3034c7077/lib/exonerate.ex#L427

but I am not experienced enough in Elixir to know if it's stylistically a poor choice to push this into the opts parameter that's already there but used for an entirely different set of options.