Add support for XMSS SSH Keys and Certificates

[geedo0]

Upstream OpenSSH has support for XMSS-based SSH Keys and Certificates ref. Equivalent and interoperable support can be implemented in the OQS fork and liboqs's XMSS implementation. It would be good to implement this if issue #24 were to be picked up.

Implementation-wise this means adding an implementation of the sshkey interface for liboqs's sig_stfl API to ssh-oqs and adding a generator entry for XMSS.