Server-side TLS support?

It looks like only bidirectional TLS is supported, since the `key` and `cert` options are non-optional fields of `TlsConnectionOptions`. Is this correct, or am I just misunderstanding how to configure a TLS connection? If only bidirectional TLS is supported, it appears to make the crate incompatible with most users of AWS MSK, because MSK only supports bidirectional TLS when configured to do so with an AWS Private CA: https://docs.aws.amazon.com/msk/latest/developerguide/msk-authentication.html