Issue description

At present, the traffic-split plugin provides an initial version. The functions that support the health check and retry mechanism will be implemented in the next iteration.

How to achieve:

Use the upstream_id method in the plugin to refer to the defined upstream so that we can reuse the health detection and retry mechanism that the upstream already has.

The PR of the traffic-split plugin: https://github.com/apache/apisix/pull/2935