Transport trait implementation #192

I'm trying to get the pubnub client to work on an esp32 board (this is a std environment but I've not figured out async yet so trying blocking APIs), where using reqwest is too much of a pain.

I think reqwest depends on openssl which I'm not sure how to get to work for my target platform.

The docs mention that you can provide your own transport implementation, but I get a 404 when I try to find it. I would like to use this trait and use the esp32 HTTP client to do the work of connecting to the pubnub API endpoints.

Is this trait gone? Wondering if there's a way to surface TLS library as a feature flag that lets you choose between say rust_tls/openssl etc.