Support http.nonProxyHosts for default proxy in NetHttpTransport

[burkedavison]

https.proxyHost and https.proxyPort are used to create a default proxy when none is explicitly provided.

Oracle documentation on this feature which is linked in our documentation also specifies behavior for http.nonProxyHosts which is not currently supported in the implementation.

Until supported, dynamic proxy behavior can be implemented with a custom ConnectionFactory. Example:

ConnectionFactory connectionFactory = new ConnectionFactory() {
  @Override 
  public HttpURLConnection openConnection(URL url) throws IOException, ClassCastException {
    return shouldUseProxy(url) ? url.openConnection(proxy) : url.openConnection();
  }
  private boolean shouldUseProxy(URL url) { 
    // Implement by parsing http.nonProxyHosts, or with direct logic. 
  }
}

HttpTransport transport = 
  new NetHttpTransport.Builder()
    .setConnectionFactory(connectionFactory)
    // Do not call .setProxy()
    .build();