server: add method for retrieving client certificates

[nacardin]

Fixes #33

[skade]

As mentioned before, I'm not interested in exposing peer certificates directly in any fashion. Direct access to certificates is a sharp tool, and gives raise to a number bugs through implementing the intended operations on them wrong.

async-tls is intended to be a higher-level library that implements these operations.

Also, please note the new maintenance notice: I will continue to update async-tls, but I am searching for maintainers taking over its library design. https://github.com/async-rs/async-tls#maintenance-notice