Support Main and Sandbox Endpoints in API YAML

Description

Gateway Config

gateway_main_host: "*.prod.wso2.com"
gateway_sandbox_host: "*.sand.wso2.com"

Simplest API YAML with just the main endpoint.

version: api-platform.wso2.com/v1
kind: http/rest
metadata:
  name: Weather-API
  version: v1.0
spec:
  context: /weather
  upstream:
    main:
      url: http://prod-backend:5000/api/v2
  gatewayHost: # optional gateway host
    main: apis.apis.prod.wso2.com
  operations:
    - method: GET
      path: /{country_code}/{city}

With sandbox endpoint

version: api-platform.wso2.com/v1
kind: http/rest
metadata:
  name: Weather-API
  version: v1.0
spec:
  context: /weather
  upstream:
    main:
      url: http://prod-backend:5000/api/v2
    sandbox:
      url: http://sandbox-backend:5000/api/v2
  gatewayHost: # optional gateway host
    main: apis.apis.prod.wso2.com
    sandbox: apis.sand.wso2.com
  operations:
    - method: GET
      path: /{country_code}/{city}

Start by locating the API YAML parsing or validation entry point and any existing handling for upstream and gatewayHost. Compare the main-only and sandbox examples; done means both endpoint forms are accepted consistently, with coverage for the configurations shown.