Support v3 PuTTY keys

[BerndSchuller]

Hi!
With the latest PuTTY, the header line has changed to "PuTTY-User-Key-File-3".
This trivial patch will parse both old and new PuTTY key files correctly.

[hierynomus]

Is that the only change? There is no format change involved?

Would it be possible to add a unit test with a new key type in there, so that we can verify this will keep on working.

[BerndSchuller]

Is that the only change? There is no format change involved?

Would it be possible to add a unit test with a new key type in there, so that we can verify this will keep on working.

yes, it's the only change. Test added.

[jpstotz]

This PR seems to enable support for unencrypted Putty v3 private keys.

But the new file format also comes with a different encryption using a Argon2 based key derivation function. This is not supported by sshj therefore it should better throw an exception if the PPK key file contains an Key-Derivation entry as even with the correct password sshj will never be able to decrypt the key.