Add XML writer

[stloyd]

Change Log

Added

• Add XML writer

Fixed

Changed

Removed

Deprecated

Security

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Description

[norberttech]

Just to give some context, @stloyd created this PR long time ago but back then I decided to not merge it because it was missing two things:

• handling deeply nested data structures lists/maps/structs
• handling attributes

The first one is a similar concept that we are using in ParquetConverter - recursive iteration over Flow entries and converting them into XML
The second one can be implemented based on naming convention, for example, if a entry has __xml_attribute__{entry_name} prefix (or something different) it would be added as an attribute to row node (or struct/list/map). A helper function would be nice, something like entry_to_xml_attribute(ref(...)).

Besides those two things, it is ready to merge. If anyone wants to take it over (not sure when I will have time, XML is not the highest priority for me right now but it's definitely important) I would kindly ask to pull the code directly from Joseph's branch so we can keep the clean history of contributions.