Add "inspection" mode to client libraries

[melange396]

Add an inspection mode (like a debug mode) to client libraries so we can "see" (and/or log) what exact http[s] requests are made during its operation.

This can help identify redundant/repeated requests, or shed light on which might be taking unexpected amounts of time to return. It can also elucidate behaviors in the covidcast client, where a single method call may be decomposed into multiple http requests.

A simple implementation of this could just print info for each request as it is made (URL, method, headers, response time, response bytes/rows, etc). An additional deeper feature could print this info, BUT return empty/dummy data and not actually make the request. The latter could be useful for testing long-running or expensive data retrieval scripts without making the user wait for results or putting load on the server (obviously, this may not be very useful in instances when later query parameters are dependent on earlier query results).