Add API support for file contents

[frm]

Retrieving the contents of a file is an API call that wasn't implemented. This PR adds it, although it's not ideal since we're not streaming the file and everything is kept in memory. I couldn't get streaming to work.

This PR also updates hackney as ssl_verify_fun has some classic issues:

openai.ex (dd48493)
❯ m compile
==> ssl_verify_fun
Compiling 7 files (.erl)
src/ssl_verify_fun_cert_helpers.erl:13:14: can't find include lib "public_key/include/public_key.hrl"
%   13| -include_lib("public_key/include/public_key.hrl").
%     |              ^