Capability of setting HTTP/HTTPS proxy as client parameter vs using ENV

## Context

I have an app that would only use https proxy for clients that make outgoing HTTP requests to external services, like ABS. I see this client does support proxy, but only as ENV variable. See Core HTTP Client: https://github.com/Azure/azure-storage-ruby/blob/05ea58af3859edda790e769be5beba7fcfb73c0f/common/lib/azure/storage/common/core/http_client.rb#L69

## Solution
We should be able to define outbound proxy without affecting the whole app environment. Passing optional client param from its instantiation and using ENV var as fallback (if any).