Provide support for ClientHandlers to use a proxy server within OpenAM (eg. for reCAPTCHA in USS)

In some scenarios, making http(s) requests to other parts of the network, for example the Internet from an internal network, requires traversing a proxy server.

For example using reCAPTCHA within OpenAM USS requires an https request to external systems and may require traversing a proxy server. Currently this is not possible.

Note that setting -Dhttps.proxyHost *or *-Dhttps.proxyPort does not work in this case as the code invoking the Apache HttpClient libraries does not call .useSystemProperties() that would have facilitated this.