This change updates the implementation of the Credential objects.
The UsernamePassword and Device credentials are now first-class
objects and, along with the base Credential, have been restructured
to match the current struct style. Comprehensive unit test suites
for each class have been added. Additionally, the credential
factory code and its usage in the KMIPProxy class and associated
test suites have been updated to reflect this change.
This implementation supports version 1.1 of the KMIP spec. The
library supports the following operations for SymmetricKey objects:
create
register
get
destroy