Mark hackney optional

[maxsalven]

The library allows you to provide your own http client, but hackney is still marked as a required dep.

[nacengineer]

I would actually recommend moving to Req instead which is an Elixir library that is maintained by the elixir core team, not that Hackney isn't maintained but is more of an Erlang first library.

[juancgalvis]

Please make it as optional dependency, declare a behaviour and allow to set our custom http client module