Add TLS support

[ryan-summers]

Add support for a TLS-based connection with the broker.

This may necessitate the development of new embedded-nal traits for the TCP stack.

embedded-tls can provide the necessary TLS components.

[teovillanueva]

Hello! Is TLS support planned?

[ryan-summers]

It is indeed planned eventually, but much of minimq development is driven by the user needs of https://github.com/quartiq/stabilizer and https://github.com/quartiq/booster. Given that we don't have any immediate need yet, it's not yet on my priority list and is more of a fun project I'd like to get to later.

cc @jordens

[Ddystopia]

Hello, I want to mention https://github.com/drogue-iot/embedded-tls here. It allows to wrap any tcp stream into tls. Maybe it would be useful for someone

[ryan-summers]

Yeah, the intent would be to leverage embedded-tls (or rustls when no-std support is complete) to facilitate this. Is your comment about wrapping a TCP stream with a TLS mechanism and then treating it opaquely as a "TCP stream"? That would alos be an interesting approach.

[overheat]

embedded-tls already support Client Certificate Authorization, drogue-iot/embedded-tls#135
Cloud we have this tls support with a mutual auth example, like AWS iot core does?