Netty client: honor https.proxyHost / https.proxyPort system properties for HTTPS requests #1570

Problem

NettyClientHelper.getProxyHost() / getProxyPort() fall back only to the http.proxyHost / http.proxyPort system properties, for both HTTP and HTTPS requests. The JDK convention (see Networking Properties) uses https.proxyHost / https.proxyPort for HTTPS. Users with only the https.* properties set get no proxy, and users with only the http.* properties set send HTTPS through a proxy they may not have meant for it.

Proposed fix

• Choose the system properties by scheme: https.proxyHost / https.proxyPort (default 443 per the JDK, or keep 3128) for HTTPS, http.* for HTTP.
• Keep the proxyHost / proxyPort connector parameters for both schemes, and possibly add httpsProxyHost / httpsProxyPort parameters.
• Make the pool and target computation (forwardProxy in create(), acquire()) use the proxy for the request's scheme.
• Update the Javadoc parameter table and add tests.