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

[scott-xu]

This PR adds support for [email protected] 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

[scott-xu]

The decompression works when receive SSH_MSG_GLOBAL_REQUEST.
The decompression stops working when receive SSH_MSG_CHANNEL_OPEN_CONFIRMATION (compared with non-compression mode). It tries to decompress byte array: [20 13 74 63 12 112 208 192 0 16]. No idea what's going on here. Thoughts? @Rob-Hague

[scott-xu]

Ready to review @Rob-Hague @WojciechNagorski

[Rob-Hague]

Nice. Is there a reason for not testing "zlib" as well?

[scott-xu]

Nice. Is there a reason for not testing "zlib" as well?

Because sshd doesn't support "zlib"(pre-auth) anymore.

[Rob-Hague]

Ok, in that case I suggest we do not support the "zlib" algorithm since we can't test it easily. What do you think?

This is good to go otherwise.

[Rob-Hague]

Thanks, this is good to merge.

[scott-xu]

@WojciechNagorski Is there any other thing I can do to get this merged? Thanks!

[WojciechNagorski]

Yes. You can ask @Rob-Hague for review. I do not have time now.

[Rob-Hague]

I approved it but it needs CODEOWNERS approval:

No rush

[WojciechNagorski]

I've added @Rob-Hague to code owners.

[scott-xu]

Perfect!