enable optimizely node sdk to work behind proxy

If sdk is meant to be initialised in air tight environment and you are meant to use proxy for security and audit reasons, then its impossible to do so with createInstance method from neither react nor node SDKs. Datafile wont be fetched, App will crash.

We need some way to avoid it as in better control network traffic. Usually its done by few methods:
* out of the box support HTTP_PROXY and HTTPS_PROXY env vars
* options in createInstance to point to the proxy
* options in createInstance to override http-agent or https-agent with custom one which supports proxy
* options in createInstance to override fetch with custom one which supports proxy