Add APIs for custom attribute definitions

Editor assessment

The subject states a real API addition, endpoints for custom attribute definitions, and feat is the right type. What is missing is any body on the contract: which operations the APIs support, what the definition schema looks like, and how the definitions relate to the attribute values already rendered, questions any integrator must answer from the diff. No scope marks the api area. The trailing pull-request number is conventional. Hygiene is good: imperative shape, no trailing period, no junk. Clear headline, absent substance.