feat: add zipkin variable

[wizhuo]

Description

I would like to expose the zipkin span information during the rewrite phase so that it can be used in other plugins and scenarios. For example, it would be useful to include the trace ID when printing logs.

Fixes # (issue)

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[monkeyDluffy6017]

Good job! Please make the ci pass

[wizhuo]

Good job! Please make the ci pass

I have already updated, please check

[monkeyDluffy6017]

@wizhuo please check the ci error

[wizhuo]

@wizhuo please check the ci error

Please advise on how to resolve the following issue

Error: nginx: [error] open() "/home/runner/work/apisix/apisix/logs/nginx.pid" failed (2: No such file or directory)
[ info ] stop -> [ Done ]

• git checkout conf/config.yaml
  Updated 0 paths from the index
• for f in ./t/cli/test_*.sh
• PATH=/usr/local/openresty-debug/nginx/sbin:/usr/local/openresty-debug/luajit/bin:/usr/local/openresty-debug/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin
• ./t/cli/test_zipkin_set_ngx_var.sh
  ./ci/linux_apisix_current_luarocks_runner.sh: line 74: ./t/cli/test_zipkin_set_ngx_var.sh: Permission denied
  Error: Process completed with exit code 126.

[monkeyDluffy6017]

@wizhuo you can test in your private repository with the following configuration

  - name: Run debugger
    if: ${{ failure() }}
    uses: mxschmitt/action-tmate@v3
    timeout-minutes: 15

[monkeyDluffy6017]

@wizhuo Nice job!