Add an API endpoint offering each dataset oemetadata in jsonld

Description of the issue

Add the content of the context document and oemetadata JSON document in a new jsonld file and offer it to the user.

Note: The context.json is currently enabling widely used JSONLD Parsers to translate the oemetadata into rdf / graph data. Still, tools like Protégé are not yet supported fully due to the incomplete nature of jsonld mappings. We would need to provide structurl/type information. This would also mean we have a duplicate Mainly used the jsonld-playground for testing. JSON-LD Python libs seem to be incomplete or need specific configuration.

Ideas of solution

• Simple improvement with low effort and good value for users.

• The jsonld mapping stored in the context.json with each oemetadata release. We just add this document to any tables metadata and offer and download endpoint & Button on the table detail page.

Workflow checklist

• I am aware of the workflow in CONTRIBUTING.md