Add support for using an HTTP proxy

Hello,

we would like to be able to use an outgoing proxy with this library (our server don't have direct internet connection). One current option is to overload `makeFetchRequest` (great design decision to have put it in its own method btw) and add the `agent` option here, but on the other hand it forces to add a direct dependency to node-fetch and maybe ties our code a bit to much to implementation details of this library. So what I suggest would be to have a function `setHttpAgent` to set a `this.httpAgent`, and then use this `httpAgent` as the `agent` parameter of `fetch` in `makeFetchRequest`.

What do you think? I can make a MR if you are okay with this idea.

Thanks