Add length details to SSH DSA/RSA Fingerprints

[claudijd]

This request is from #476, but I wanted to call it out as it's own thing.

Basically, right now, we will provide the following fingerprint information if the host allows us to get it...

"fingerprints": {
"rsa": {
"known_bad": "false",
"md5": "fc:c5:5b:0d:f0:c6:fd:fe:80:18:62:2c:05:38:20:8a",
"sha1": "e1:3c:71:49:80:37:87:32:b5:0c:e3:86:41:ef:2e:2a:2f:14:e3:58",
"sha256": "68:7d:30:37:67:2c:eb:1e:4a:b5:ff:4f:bc:8c:e8:41:e1:55:a8:30:42:e0:8e:3c:e3:0a:ba:bc:db:fd:5c:50"
}
}

What would be most useful is denoting the length of the key, so automated frameworks or even policy frameworks can pin to that and flag it if it's below a desirable minimum length.

[claudijd]

@exploide

[claudijd]

As I look at the fingerprint implementation, I'm beginning to believe it's not correct.

I took at look at some prior art here (https://github.com/bensie/sshkey), which seems like it could help make better use of the public keys offered in the DSA/RSA exchanges and make the information actually correct, so I think that's a good place to either look or just implement sshkey into ssh_scan as a dependency or just replicate the OpenSSL logic to decode the pub keys and present useful information about them.

[claudijd]

I have a PR (#484) to fix this