Possibility to hand over parameters to underlying libraries (e.g. EventSource)

[ThomasHawel]

Is your feature request related to a problem? Please describe. I had a problem receiving messages with the payments.stream object, in detail I got "self signed certificate in certificate chain". This error occured when I was working in my companies newtwork. The solution for me was, to directly change the code and added the proxy information in "node_modules/stellar-sdk/lib/call_builder.js". The modification (line 56) es = new EventSource(_this.url.toString(), {proxy: 'http://...proxy server...'}); solved the issue.

Describe the solution you'd like Would be nice to have the possibility passing the proxy parameter to this function from stellar-sdk directly.