grpc-js: Add HTTP CONNECT support, i.e. egress proxy support

[murgatroid99]

The implementation here is based pretty closely on the corresponding core code in https://github.com/grpc/grpc/blob/master/src/core/ext/filters/client_channel/http_proxy.cc. The code for establishing the connection is loosely based on this section of the tunnel-agent package, which is used by the request package for establishing HTTP CONNECT tunnels.

With this change the library handles the environment variables grpc_proxy, https_proxy, http_proxy, no_grpc_proxy, and no_proxy.

I am considering this an experimental feature for now because I am not familiar with environments with those kinds of proxies and I don't know how to test it.

[murgatroid99]

This PR is currently not correct. The socket option that I use is actually only used by tls as the underlying connection. The http2 module does not use it at all, so this implementation will not use the proxy for plaintext connections.