Add @Throttle and @EndpointRateLimit decorators to the exchangeToken
endpoint matching other auth endpoints (20/hour per throttler, 5/60s
per IP via EndpointRateLimitGuard). Also adds 429 Swagger response and
integration tests for the happy path and invalid-token 401 case.
Co-Authored-By: Paperclip <noreply@paperclip.ing>