This commit is groundwork for full certificate support, but doesn't
complete the job by itself. It introduces the new key types, and adds
a test in cryptsuite ensuring they work as expected, but nothing else.
If you manually construct a PPK file for one of the new key types, so
that it has a certificate in the public key field, then this commit
enables PuTTY to present that key to a server for user authentication,
either directly or via Pageant storing and using it. But I haven't yet
provided any mechanism for making such a PPK, so by itself, this isn't
much use.
Also, these new key types are not yet included in the KEXINIT host
keys list, because if they were, they'd just be treated as normal host
keys, in that you'd be asked to manually confirm the SSH fingerprint
of the certificate. I'll enable them for host keys once I add the
missing pieces.
