Added signature verification support for SMB2 client

[cri-triovega]

I added the signature verification for SMB2 messages on client side.

Adding the attribute m_sessionSetupResponseMessage to SMB2Client was a bit lazy, but the easiest method to implement the check without changing to much of the library code. Maybe you have a better approach in mind @TalAloni ?

Tests prove that the manipulation of the signature result in the desired behavior (either discarding the invalid message or disconnect from the server).

[TalAloni]

First of all, thanks for the contribution!
My approach in software is "make it work" and I admit that the SMB2 client is very lax in terms of security.
and I admit that having the option to enable signature checks is in order.

With that said, I need to carefully check the implementation to make sure it does not introduce any regression,
and the selection of integration tests instead of unit tests is also questionable,
and I would probably move some of the signature verification code to a new method in SMB2Cryptography.

I will try to find time for a deeper review in the coming weeks. Thanks again.

[TalAloni]

Apologies for taking such a long time to get to this. I started to look into it today and a deep dive into the specifications is in order to ensure that the code behaves correctly in all cases so I haven't been able to cover everything yet.

1. I would probably prefer an SMB2Client opt-in constructor argument to activate signature requirement and verification, something like 'requestSigning'.
2. I added SMB2Cryptography.VerifySignature method that should help with the verification.
3. IIUC sending SecurityMode.SigningRequired | SecurityMode.SigningEnabled is incorrect, SigningEnabled is implied when SigningRequired is set.