examples: require good code to show how to support both secure and insecure connections

[barsnick]

An mentioned in #597, #706 and #745, the example clients could make use of good code to show how to support both secure (wss://) and insecure (ws://) from the same implementation. I'll try to make this ticket "meta" (excuse me if it disturbs the flow.)

[barsnick]

I chose to work with the debug_client example, which was already made to be switched between the two by changing only a few lines of code. I managed to make it work with both, with as much common code as possible, by using templates.

It's not a complete example, and it doesn't even work for the given echo server, as you will see in the commit message of the pull request:

This change does not cover proper configuration of the TLS connection. A proper implementation requires more code for choice of protocols, verification of certificates, and so on.

Most notably, the given default URI wss://echo.websocket.org currently does not even work with this example, because it makes use of a wildcard certificate. This requires more asio boilerplate.