Encryption with direct+HKDF algorithms is not implemented

[BrianSipos]

This issue tracks a NotImplementedError when attempting to use at layer-1 either algorithm DirectHKDFSHA256 or DirectHKDFAES256 with recipient class DirectEncryption. The correct behavior is to have the DirectEncryption.compute_cek() return the CEK for Direct algo and run HKDF for the direct+HKDF algos.

pycose/pycose/messages/recipient.py

Line 239 in 1458ddf

raise NotImplementedError()