sftpd: add support for SSH user certificate authentication

This add support for PROTOCOL.certkeys vendor extension:

https://cvsweb.openbsd.org/src/usr.bin/ssh/PROTOCOL.certkeys?rev=1.8

Fixes #117

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

sftpgo.json

@@ -16,6 +16,7 @@
     "kex_algorithms": [],
     "ciphers": [],
     "macs": [],
+    "trusted_user_ca_keys":[],
     "login_banner_file": "",
     "setstat_mode": 0,
     "enabled_ssh_commands": [