Fetch JWK from HTTP with reload feature

[pouriya]

I'm thinking of using jwt_key option. if it starts with http:// or https:// then it's not a filename and we can fetch key from that http address (via httpc). Also if a new option jwt_fetch_interval is set, start a new gen_server in ejabberd_auth_jwt:start/1 to update JWK periodically.
in future we can add more options (e.g. http method, ...).