Support explicit imports of xml documents into trestle project via conversion to json.

[butler54]

Challenge.

• External entities may produce only xml documents
• Trestle is anchored w.r.t json/ yaml
• xml-> json conversion requires less than easy to install libraries

Solution

• Trestle only to support XML via an explicit import which does a conversion.
  e.g. trestle xml-import -f path_to_xml.xml -o my_catalog this needs to be part of an external plugin (e.g. the Fedramp extension).

Completion criteria

• xml-import is part of compliance-trestle-fedramp
• xml-import can pull from remote environments using caching / remote functionality in trestle
• Add to core trestle documentation in error handling to notify users if an xml OSCAL document is encountered an point to trestle doc page on what to do
  • Example scenario: json ssp refers to xml profile. In this case we can error with message on what you should do.

[butler54]

@vikas-agarwal76 - can you confirm the current state for xml import? I believe we are (at a minimum) missing the docs.

[jpower432]

The oscal-cli fulfills this requirement