Allow for a custom hostname validator

From @ivanitskiy:

here is an idea for you. let's have a callback on the client so users/developers can build their own validators (say they want to have allow list). from: https://github.com/auto-ssl/lua-resty-auto-ssl

auto_ssl:set("allow_domain", function(domain, auto_ssl, ssl_options, renewal)
  return ngx.re.match(domain, "^(example.com|example.net)$", "ijo")
end)

we can provide a default callback but allow people to make their own validations.