We missed this in the original interface specification, but the `grant_type=authorization_code` requires it, per RFC6749 (https://tools.ietf.org/html/rfc6749#section-4.1.3).
Signed-off-by: Matt Moyer <moyerm@vmware.com>
Because we want it to implement an AuthcodeExchanger interface and
do it in a way that will be more unit test-friendly than the underlying
library that we intend to use inside its implementation.
