Health Check Http Status Code In Events

Added http_status_code field to :ref:HealthCheckEjectUnhealthy <envoy_v3_api_msg_data.core.v3.HealthCheckEjectUnhealthy> and :ref:HealthCheckFailure <envoy_v3_api_msg_data.core.v3.HealthCheckFailure> proto messages. When the HTTP health checker receives a non-2xx response, the HTTP status code is now populated in health check events. A value of 0 indicates that no HTTP status code was recorded (e.g., network-level failures or non-HTTP health checkers such as TCP, gRPC, Redis, and Thrift).