add variable from secret when applying istio cr

[2456868764]

Ⅰ. Describe what this PR did

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 52.00000% with 36 lines in your changes missing coverage. Please review.

Project coverage is 43.47%. Comparing base (ef31e09) to head (d70e5a2).
Report is 346 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/ingress/config/ingress_config.go	12.90%	27 Missing ⚠️
pkg/ingress/config/ingress_template.go	79.54%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1877      +/-   ##
==========================================
+ Coverage   35.91%   43.47%   +7.55%     
==========================================
  Files          69       77       +8     
  Lines       11576    12374     +798     
==========================================
+ Hits         4157     5379    +1222     
+ Misses       7104     6659     -445     
- Partials      315      336      +21     

Files with missing lines	Coverage Δ
pkg/ingress/config/ingress_template.go	79.54% <79.54%> (ø)
pkg/ingress/config/ingress_config.go	13.01% <12.90%> (-0.87%)	⬇️

... and 70 files with indirect coverage changes

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[johnlanni]

这个函数我建议抽象成 getValue，把从什么来源获取的逻辑也包在里面，例如可能未来会支持要从 nacos 获取某个配置项的值

[2456868764]

OK