GatewayFlowRule can not use multiple paramItem

As we konw，if we set paramItem in GatewayFlowRule，it will used as ParamFlowRule。
but ，in ParamFlowRule，we can set multiple paramItem by paramFlowItemList

How to use multiple paramItem in GatewayFlowRule。

使用网关流控时，如果要使用参数限流配置，没法和热点参数限流一样，添加多个热点参数项。
比如，在网关里面配置route_id=A，按host=a.com限流100qps，host=b.com限流50qps

Start by comparing how GatewayFlowRule uses paramItem with how ParamFlowRule handles paramFlowItemList; no specific files or tests are named in the issue. Define support for multiple parameter items in gateway flow control, including separate limits for host=a.com and host=b.com under route_id=A, then add coverage for that behavior.

Written by the indexing model from the issue text.