Add support HTTP-proxies and encryption with SSL

[zuenkoma]

Type of change

• Bug fix
• New feature

Description

Implemented full support for HTTP/HTTPS proxies and SSL-encrypted connection to proxy servers (HTTPS, SOCKS4S, SOCKS5S). Previously the application worked only with plain SOCKS4/SOCKS5.

How Has This Been Tested?

• Manually tested with public HTTP, HTTPS and SOCKS5S proxies.
• Verified that proxy check correctly identifies alive proxies of each type.

Checklist:

• My code follows the style guidelines of this project.
• I have added comments to my code in more complex areas.
• I have tested the code in both development and production environments.

[Big-Iron-Cheems]

I have some questions regarding the PR if you could address them @zuenkoma.

• In ConnectionMixin you do:

pipeline.addFirst(new HttpProxyHandler(...));
pipeline.addFirst(new SslHandler(engine));

So the pipeline becomes SslHandler -> HttpProxyHandler -> ...
Is this order correct or should the SSL step happen after the HTTP one?

• InsecureTrustManagerFactory.INSTANCE disables certificate validation, is this necessary or can we use the default?
• Current check only verifies response starts with HTTP/, meaning failures like 403, 500, or malformed proxy responses still count as alive. Perhaps we should parse the HTTP status line and require a 200 for succesful CONNECT support?
• CONNECT target uses 0.0.0.0:80, maybe using a real reachable address like 1.1.1.1:80 could be better?
• Perhaps tls, ssl, or encryptedTransport would be a better name than secure.

Thanks for the contribution nonetheless.