feat: client tls_options/sockopts spec, check ssl and tls

[MarkoMin]

Just recently I tried to use this lib to send some mails through existing SMTP server and I've stumbled upon few things:

1. Correct me if I'm wrong, but based on this reply and my own experience, it's not possible to establish a connection to SMTP server when both ssl and tls options are set. We must choose only one of them. That's why I've added chech_ssl_optionss/2 When ssl is set to true and tls to always I get {missing_requirement, Host, tls} error. When I choose only one, sending succeeds.
2. sockopts can be [tls_client_options()] when ssl is set to true. This doesn't help very much to dialyzer, but it does to the documentation.
3. tls_options specified more correctly (guess that those types weren't exposed when defining gen_smtp_client:options/0 type.)