[CXF-9218]Respect configured ProxySelector instead of hard-wiring sys…

[ffang]

…tem proxy properties

Summary

ProxyFactory was reading http.proxyHost, http.proxyPort, and http.nonProxyHosts
directly from system properties at construction time, bypassing Java's ProxySelector
mechanism entirely. This prevented applications from using custom ProxySelector
implementations — for example, wrapping sun.net.spi.DefaultProxySelector to selectively
bypass the proxy for specific hosts.

The fix removes the system-property reading from ProxyFactory and delegates to
ProxySelector.getDefault() instead. When no custom ProxySelector is installed the
behaviour is identical to before (DefaultProxySelector reads the same system properties
internally). When a custom ProxySelector is installed it is now consulted as expected.

The explicit HTTPClientPolicy.setProxyServer(...) path is unaffected.

Behaviour notes

• Default behaviour unchanged — DefaultProxySelector reads http.proxyHost /
  http.proxyPort / http.nonProxyHosts exactly as before.
• Both HttpClientHTTPConduit and URLConnectionHTTPConduit benefit from this fix.

[reta]

Much simpler, thanks @ffang !

[ffang]

Thanks @reta !