fix(traffic-split): binding upstream via upstream_id is invalid

[Firstsawyou]

What this PR does / why we need it:

fix #3740
Related issues: #3787

Because the current traffic-split plug-in supports upstream_id to refer to the upstream by rewriting the value of ctx.matched_route.value.upstream_id. However, when the upstream_id is used by the route and the plugin at the same time, the rewriting of ctx will cause the upstream_id on the original cached route to be lost.
Therefore, this PR creates a temporary variable of ctx.upstream_id to cache the upstream_id configured in the plug-in, avoiding the modification of ctx.matched_route.value.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?
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