Add support for HTTPS proxy

[kelunik]

Can be used with php leproxy.php 'https://127.0.0.1:5512?local_cert=/home/kelunik/GitHub/amphp/http-server/tools/tls/localhost.pem'

See https://github.com/amphp/http-client/blob/58c0c19767cb6ade95f29983f0cb540a83dde752/examples/basic/8-proxy.php for a client implementation.

[clue]

@kelunik Thank you for looking into, this looks interesting!

There's also #53 which discussed always enabling TLS by default and making encrypted and unencpryted connections work on the same port. What do you think about this?

[kelunik]

@clue There have been vulnerabilities and weaknesses due to one port being used for both. I think these were mostly due to the client using opportunistic TLS, so they might not be applicable if the client knows whether it wants to use TLS or not. Maybe @paragonie-scott has some input here.

[kelunik]

@clue Even if both might be used at the same port in the future, there should be a way to force TLS being used.

We can add same port plain and TLS later using http+https as scheme or similar.

[kelunik]

@clue What's required to get this in?

[clue]

@kelunik Thanks for the friendly reminder. I'd rather not rush this and break this feature again in the future. Instead, I'd like to properly address the underlying feature request in #53 (which in turn depends on reactphp/socket#89 and possibly others).

Rest assured, this is definitely on the roadmap and I'm planning to pick this up as soon as time permits. In the meantime, you can always keep using your forked version.

I hope this helps 👍