Add support for using a remote Chrome instance for PDF export

ATM we're supporting two modes:

• XWiki manages the Chrome Docker container (pulls the right Docker image, creates the container, starts, stop and removes it)
• XWiki reuses a local Chrome Docker container

The goal of this issue is to add a third mode:

• XWiki connects to a remote Chrome instance (running inside Docker or not, doesn't matter)

This implies adding a new configuration option to specify the host / IP address of the remote Chrome instance.