full support for encrypted PuTTY v3 files

[jpstotz]

Based on my patch yesterday I have implemented full support for encrypted PuTTY v3 keys.

Edit 2: Finally thanks to BouncCastle Argon2 implementation no external dependencies and interface is needed anymore. Unit test is in place and works

[jpstotz]

@hierynomus Would be acceptable to add the unit test shown in my first comment?
The library testImplementation group: 'de.mkammerer', name: 'argon2-jvm', version: '2.11' contains native libraries for Linux (x86, x86_64, arm, ar64) Windows x86/x86-64 and MacOS x86_64 and arm64.

If we would include this dependency the unit tests would only be executable on these platforms by default.

[exceptionfactory]

@jpstotz SSHJ currently includes a dependency on Bouncy Castle, which includes an Argon2 implementation, what do you think about providing an implementation based on the Bouncy Castle class Argon2BytesGenerator?

@hierynomus has mentioned attempting to avoid unnecessary dependencies on Bouncy Castle, but perhaps this is worth including?

[jpstotz]

@exceptionfactory Thanks, I did not know that BouncyCastle already contains an Argon2 implementation. That really simplifies everything.

[jpstotz]

I think this PR is finally complete and ready to merge.

The unit tests now cover all the variants and they verify that the keys has been loaded correctly.