RDP proxy server-side Kerberos support

[TheBestTvarynka]

Hi,
I added server-side Kerberos support for the RDP proxy feature. I tried to make as few changes as possible.

The Kerberos server feature is unstable, so it will work only if you enable the enable_unstable in config file and provide the Kerberos server configuration. Here is an example of my cofig:

  "__debug__": {
    "enable_unstable": true,
    "kerberos_server": {
      "max_time_skew": 64,
      "ticket_decryption_key": [105, 224, 208, 190, 41, 253, 44, 134, 64, 29, 178, 11, 37, 172, 124, 169, 48, 202, 121, 255, 218, 220, 247, 114, 141, 182, 249, 108, 60, 251, 145, 17]
    }
  }

If the enable_unstable is not enabled, then the Kerberos server feature will not work and NTLM server will be used (even if you provide the Kerberos config).

[CBenoit]

issue: The absence of the domain field is intentional, we decided to focus on supporting only the FQDN/UPN format, e.g.: username@domain via the username field.
This is handled cleanly by sspi::Username::parse.