XML Loader/Dumper

[multimeric]

What is your feature request?
Add an XML loader/dumper. This could be a nice counterpart to the XML Schema Definition Language (XSD) import I have provided in linkml/schema-automator#153, although the loader/dumper should support any LinkML schema, not just one generated by schema-automator.

The main challenge would be in handling the attribute/element distinction when dumping. I think it should check some part of the schema to know which to use (currently my PR uses keywords), and otherwise defaults to using elements always.

How important is this feature to you? Select from the options below:
• Medium - can do work without it; but it's important (e.g. to save time or for convenience)

Additional context
As usual, happy to submit an implementation if this feature request is approved.