Add support for Zlib compression (.NET 6.0 onward only)

[scott-xu]

This PR adds support for zlib@openssh.com described in https://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt

Resolves#1130
Resolves#559
Resolves#336
Resolves#119

Notes:

1. This PR only adds compression support for .NET 6.0+.

2. "none" is the first and preferred compression method which means no compression by default, just like ssh_config:

   Compression
   Specifies whether to use compression. The argument must
   be yes or no (the default).

   The description of ssh -C option explains the reason:

   Compression is desirable on
   modem lines and other slow connections, but will only
   slow down things on fast networks.

3. zlib (pre-auth) is not supported in OpenSSH server (sshd) but is listed in OpenSSH client (ssh).
   Since it is not easy to do integration-test for zlib (pre-auth), this PR does not add zlib (pre-auth).

[scott-xu]

With this PR, SSH.NET targeted to frameworks below .NET 6.0 can easily use 3rd party library to do the compression. For example: https://github.com/scott-xu/SSH.NET.Ionic.Zlib/blob/main/src/SshNet.IonicZlib/ZLib.cs