smtp_server: add support for using SNI to resolve certs

[wez]

This is a completely untested and undocumented attempt at hooking up the use of SNI to resolve the server certificate setup for an SMTP session.

Usage should be along these lines:

kumo.on('smtp_server_get_dynamic_tls_parameters', function(listener, conn_meta, client_hello)
  local hostname = client_hello.server_name

  if not hostname then
    -- SNI not provided by client, just use overall parameters
    return
  end

   -- TODO: sanity check hostname here; it was provided by the peer
   -- so treat it as untrusted. In particular, you should sanitize
   -- it before using it for path resolution!

   return {
     -- Use the SNI-provided name as the hostname to validate against
     hostname = hostname,
     tls_private_key = {
       vault_path = string.format('keys/%s.key', hostname),
     }
     tls_certificate = {
       vault_path = string.format('keys/%s.cert', hostname),
     }
   }
end)