[FEAT]: Add support for Features and Entitlements endpoints #435

The Lago API exposes Features and Entitlements (features and entitlements tags in the OpenAPI spec), but lago-python-client (at least 1.56) has no resource for them. Every other resource is available in the client, so these endpoints can only be called by hand with requests/httpx.

Missing

• Features: create, list, retrieve, update (PUT /features/{code}), delete
• Plan entitlements:
  • POST /plans/{plan_code}/entitlements (replaces all entitlements)
  • PATCH /plans/{plan_code}/entitlements (partial update)
  • DELETE /plans/{plan_code}/entitlements/{feature_code}
• Subscription entitlements, including
  DELETE /subscriptions/{external_id}/entitlements/{feature_code}

Related

• feat: Add the API fields missing from wallets, customers and more #431 adds entitlements to the plan and subscription responses (read only). It does not add the endpoints above.
• docs(features): clarify feature update and subscription entitlement removal semantics lago-openapi#615 notes that the Python client has no features or entitlements resource.