feat: the traffic-split plugin supports upstream_id

[Firstsawyou]

What this PR does / why we need it:

close #3094

How to implement the health check and retry mechanism:

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.

Reasons for the init.lua file to adjust the execution order of plugins:

Since the upstream_id in the init.lua file is processed before the plugin is executed, it will cause the upstream_id setting in the traffic-split plugin to take effect. Therefore, this PR adjusts the logic code executed by the plug-in in the init.lua file before the processing of upstream_id.

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[moonming]

What is the relationship between this pr and #3094? I didn't see it

[Firstsawyou]

What is the relationship between this pr and #3094? I didn't see it

The feat of #3094 needs to be realized by supporting upstream_id in the plugin, and this pr is to solve the problem that the plugin supports upstream_id.

[moonming]

I still don't know why health check and retry mechanism are depends on upstream id？

[Firstsawyou]

I still don't know why health check and retry mechanism are depends on upstream id？

Use the upstream_id method in the plugin to refer to the upstream that has been defined, so that we can reuse the health checks and retries that upstream already has, instead of repeatedly implementing them in the plugin.

[moonming]

I still don't know why health check and retry mechanism are depends on upstream id？

Use the upstream_id method in the plugin to refer to the upstream that has been defined, so that we can reuse the health checks and retries that upstream already has, instead of repeatedly implementing them in the plugin.

These should be explained in detail as much as possible in the issue and PR