[META]Make XML output more configurable

Problem/Motivation

This is a meta issue for tracking the over-arching improvements to VDE XML output handling, I'll be adding child issues and referencing them in the summary here too:

• #2886357: Changing default "<response>" and "<item>" tags in XML export
• #3522424: Allow removing the root XML node entirely
• #3522427: Allow adding attributes to the root XML node
• #3522429: Allow adding arbitrary XML to XML file
• #3522431: Support removing empty values from the XML output.
• #3522432: Allow adding an extra wrapper with an arbitrary name in XML exports
• #3522528: Allow specifying the XML encoding property
• #3523386: Allow pretty-printing XML