[Private Key] Add support for PuTTY private key file format (V3 and V2)

[scott-xu]

This PR adds support for PuTTY private key file format (V3 and V2) described in https://tartarus.org/~simon/putty-snapshots/htmldoc/AppendixC.html

Close#341
Close#431
Close#434
Close#510

Notes:

1. PuTTY private key file format V1 is not included because

   PPK version 1 was a badly designed format, only used during initial development, and not recommended for production use.

2. The Argon2 algorithm is from latest BouncyCastle beta version. It may subject to change, then we need to change accordingly. (Hope not). Also hope BouncyCastle makes a formal release before SSH.NET next release. Update: BouncyCastle 2.5.0 is released.

[scott-xu]

ED25519Key class does not have a constructor with bothpublicKeyData and privateKeyData. The current implementation is to generate public key from private key. Should we add a new constructor?

SSH.NET/src/Renci.SshNet/Security/Cryptography/ED25519Key.cs

Lines 98 to 104 in 3ec45e1

	publicED25519Key(byte[]privateKeyData)
	{
	PrivateKey=newbyte[Ed25519.SecretKeySize];
	PublicKey=newbyte[Ed25519.PublicKeySize];
	Buffer.BlockCopy(privateKeyData,0,PrivateKey,0,Ed25519.SecretKeySize);
	Ed25519.GeneratePublicKey(privateKeyData,0,PublicKey,0);
	}

[Rob-Hague]

it seems ok as it is, but I don't mind

[Rob-Hague]

lgtm, will wait for a stable BC release

[Rob-Hague]

it seems ok as it is, but I don't mind

[Rob-Hague]

ooh a flaky test