Support user-supplied requests.Session object for HTTP/S requests

[chuckwondo]

The requests library is used for HTTP/S requests. Default session objects are used for every request (implicitly), which not only does not allow use of custom sessions, but also may negatively impact performance to some extent because there are places where multiple requests might occur, but no session is shared across the requests, thus connection pooling is not leveraged.

Support should be added for supplying a requests_session kwarg to the MAAP initializer so that the supplied session can be used for all requests. By exposing such functionality, a user may also configure Retry capabilities. Further, we can supply sensible default retry capabilities so users don't have to configure retries if the defaults suffice.