This commit enables the optional injection of the requests.Session
object wrapped by RestSession. Operations teams have use cases for
modifying the underlying Session behavior, chief of which is to
tweak the HTTP retry policy. Currently, we're monkey-patching
our own Session object into the RestSession instance; with this PR,
we (and other teams) would be able to perform the same thing
without violating software contracts.I can share code examples of
how and why we need to do this.

Author: Andrew Schenck <aschenck@cisco.com>