ktls: alert/handshake data

[toidiu]

main...toidiu:s2n-tls:ak-ktls0_main1 is some sample code, warning.. its nasty looking code. The relevant files are s2n_shutdown.c and s2n_ktls_io.c.

This task should come after #4168 and might influence that task.

Receiving Alerts:
I'll list a few assumptions and things we should watch out for Alerts:

• we can recv an alert at any point
• kTLS could return a fragmented alert (alerts are 2 bytes but could it return 1 byte)? I was not able to repro this and verify if its something we need to worry about. Other impls dont seem to worry about this.
• If we recv alert when trying to recv app data, we need to process it and then 'rewind the read' so that we do not return the alert to the user
• when calling s2n_ktls_recv from shutdown, we only care about alerts so we drop app data
  • we will need to read into our own library owned buffer (what size should it be?) (which stuffer? conn->alert_in or conn->in)
  • since its possible to recv app data the stuffer should be >2 but also large enough to process some app data quickly
  • do we want to allocate the stuffer each time or alloc once and keep it around?
• It might be best to call s2n_process_alert_fragment

Receiving Handshake:

• We error on hadnshake message except some logic around TLS_HELLO_REQUEST and conn->config->renegotiate_request_cb. Here is some sample code but one should double check the current TLS impl also to make sure there is nothing missing.
• It might be best to call s2n_post_handshake_recv