[Feature Request] Support ignoring invalid server certificates

[cretz]

Is your feature request related to a problem? Please describe.

In Go we can use InsecureSkipVerify on tls.Config and in Java we can call setUseInsecureTrustManager, but there is no equivalent in Core.

Describe the solution you'd like

We need some kind of ignore_invalid_server_cert boolean. I don't think we want to just call it insecure because that implies a lot. And I could see one day people want more control over which errors they ignore on invalid server, but I think this would solve most needs for now.

Other users wanted to do similar, e.g. hyperium/tonic#891 (comment). https://github.com/hyperium/tonic/blob/master/examples/src/tls/client_rustls.rs is an example of custom TLS. We might have to use something like Endpoint.connect_with_connector and make our own connector. Or maybe we can upstream something to tonic.