Support QueryContextAttributes SECPKG_ATTR_SESSION_KEY

Would be great to support `SECPKG_ATTR_SESSION_KEY` with [QueryContextAttributesW](https://learn.microsoft.com/en-us/windows/win32/api/sspi/nf-sspi-querycontextattributesw) to retrieve the generated session key for the security context. This is needed if you ever want to implement support for SMB authentication as it uses this key to derive the subsequent keys it uses internally.