Create API key for merchant business (Service Provider API)

Onboards a new merchant business on behalf of a service provider. This endpoint creates a new user, business, and establishes the service provider relationship. **Use Cases:** - Service providers (like Payme) can onboard merchants through their own platform - Automates the business registration process - Establishes the service provider relationship automatically **What This Endpoint Does:** 1. Creates a new user account (if phone number doesn't exist) 2. Creates a new business with the provided information 3. Sets up the business configuration (enables API access) 4. Creates a service provider business member linking the service provider to the merchant 5. Sets referral code if provided **Important Notes:** - The authenticated business must be a service provider (checked via middleware) - Phone number must **not** already be associated with any existing user in Jeko. If a user with this phone number already exists (regardless of their onboarding status), the request fails with an error. - The service provider business is automatically linked to the merchant business - API access is automatically enabled for the new merchant business **Authentication:** Requires API key headers. The authenticated business must be a service provider.