Add skip_ssl_validation flag and refactor TLS config

[madchicken]

This PR adds the possibility of skipping client-side TLS certificate validation.
In the previous version, a user could pass a CA certificate to make an SSC connection work with a self-signed certificate. With this modification, a user can choose to skip the validation completely and avoid passing a valid certificate.

Example of usage:

let config = ConfigBuilder::default()
            .uri("127.0.0.1:7687")
            .user("some_user")
            .password("some_password")
            .skip_ssl_validation()
            .build()
            .unwrap();

[knutwalker]

Thanks, looks good!