Http::getProxy() method to get proxy configuration

MediaWiki currently uses two sources for proxy configuration:
	* the $wgHTTPProxy global configuration variable
	* the http_proxy environment variable

The HTTP proxy adress to use is a valuable information for
extensions handling directly HTTP requests instead of use
helper classes provided by the core to construct them.

This change offers an Http::getProxy() utility method to get
the configuration, regardless of the source.

Bug: T117954
Change-Id: I5df31845df71f05ac581f532cc9bd7a1fea25583