1. Add a request handler in ejabberd_http and explain how to configure the http listener so that the challenges can be solved.
2. Make acme configuration optional by providing defaults in ejabberd_acme.
3. Save the CA that the account has been created in so that it creates a new account when connecting to a new CA.
4. Small spec change in acme configuration.
This is done so that possible future updates to the data structure don't break existing code.
With this change it will be possible to update the data structure and keep the same old persistent data file, which will still have the expected list format but with more properties
