update config to support custom http client or http.RoundTripper

[ecasilla]

I wanted to add request tracing for external calls. But the client library does not support augmenting the http.Client. Would it be possible to A. Allow the users to send a custom *http.Client with a modified round tripper OR B. Allow users to Implement the http.RoundTripper interface and send that as part of the config. Then client merges that into the http creation flow.

[prayansh]

@ecasilla this is an interesting feature. Is this a common pattern among go libraries (i would really appreciate any examples)? If yes, then I would be happy to add this enhancement :)