[core-http] Consider switching from tunnel to another proxy agent library

[daviwil]

While working on PR #6772 I had a hard time figuring out how TLS settings get propagated from tunnel's options types down to the underlying HTTP(S) connections. While looking into this, I happened across the node-https-proxy-agent library which is simpler, clearer, and seemingly more robust. I think it would be wise to switch to it (or another alternative) since tunnel is currently at a not-confidence-inspiring version 0.0.6 and seemingly not actively maintained.

One concern @bterlson raised offline is whether other proxy libraries like node-https-proxy-agent support HTTPS over HTTPS proxy connections; based on what I see in the README.md of the repository, this seems to be supported.

@amarzavery - Do you remember why tunnel was chosen over other options for ms-rest-js?

/cc @southpolesteve @xirzec

[southpolesteve]

FYI We recommend https://github.com/TooTallNate/node-proxy-agent to Cosmos users. Same author as the one you linked above.

[xirzec]

core-https uses node-proxy-agent. I don't think it's worth going back and modifying the existing core-http though, so I'm going to close out this issue.