Add xsd (XML Schema) generator

[cmungall]

indirectly, from a question from @johanneswerner to NMDC

Are there plans to write an xml-schema adapter?

I don't think this is in the critical path for the current linkml plans, but we would welcome any community contribution here

[cmungall]

one possibility is linkml->jsonschema->xsd, but the latter step does not seem well supported:

https://stackoverflow.com/questions/48376526/generate-xml-schemaxsd-from-json-schema