Add support for PuTTY-User-Key-File-3

[burtek]

New version of PuttyGen saves private keys with header PuTTY-User-Key-File-3 which is not recognized by tabby-ssh module:

Example key:

PuTTY-User-Key-File-3: ssh-rsa
Encryption: none
Comment: rsa-key-20211214
Public-Lines: 6
AAAAB3NzaC1yc2EAAAADAQABAAABAQCQWPBUsyVmi5vl8myEQ91IP2JroWP1N2NR
xvFeIcGgtPSwq/7MIq2Vis8s2JlpBjFY2aDDGSlrWrh4Ryqs9NnvueLgeWjgD8wv
3qnYgv4pdu6k7HLrE5J22vQ6M8tFjpPDAVS+mfFIe9+RutO1eimhuh824ErTPKPT
LdBrheuZpUIaWl45Kd529V6CO0+b0/U/7zVfC/uKcHxDZ4RK2AkKOYMROb93B0qY
6n4t0tjkzFDJvqgMa7FoIwLkczyGSbpmKh39pvoMOyQB50WFSXbPuNscubGcOJAa
1SARXLLQh6UB9AiJqt+bk6r4OQyiXeKkk5u9lXNnxk1aMQQW46hh
Private-Lines: 14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Private-MAC: a172468747b16c488815cb90b4359fe61d262c9a9329f60cabf6763deb638d8e

(never used, I know what I'm doing pasting it here)

I see you're using sshpk which has not been updated in 3 years so the support probably won't be added for v3... Would be great to have support for it though. Also, created issue TritonDataCenter/node-sshpk#78

[burtek]

For now will be forcing PuttyGen to v2