As a user I want to be easily able to know what value type a property has

Acceptance criteria:

• The value type of each data type is exposed in the API through the endpoint /wikibase/v0/property-data-types
• Endpoint will return a JSON object that maps the property data types to the value types. Response looks like:

{"data type" : "value type"}

Task breakdown

• add to OAS
• Implement the new route
  • use DataTypeDefinitions::getValueTypes() directly in the route handler
  • no use case needed
  • don't need to add it to RouteHandlersTest
  • phpunit test may or may not be useful. whoever picks the task can decide.
  • e2e test
• spec tests
• mark production ready