support conf varible override

[quanbisen]

Please answer these questions before submitting a pull request, or your PR will get closed.

Why submit this pull request?

add conf override pass system environment with prefix AD_, such as AD_Conf_Etcd_Username=root1 override etcd user.

What changes will this PR take into? add PathExist utils and conf override statement.

Related issues

fix/resolve #2514

Checklist:

• [x] 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?
• [x] Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[bzp2010]

Hi, @quanbisen

Thank you for your contribution, but I reviewed what was mentioned in the issue and our goal is actually to work something like this https://github.com/bzp2010/apisix-dashboard/blob/8a20a9c564dc4d1953a686077f5e0c9314b86935/api/internal/config/config.go#L97-L100

It is very simple and allows us to override a single configuration item with a variable like AD_SERVER_HTTPHOST instead of overriding the entire configuration with the entire configuration file passed in by the environment variable.

Would you like to change it?

[quanbisen]

Oh, I will make it.

[quanbisen]

Hey, I have recommited. Please review my submited code.

[codecov-commenter]

Codecov Report

Merging #2587 (07b8679) into master (b777d99) will decrease coverage by 2.00%. The diff coverage is 70.00%.

@@            Coverage Diff             @@
##           master    #2587      +/-   ##
==========================================
- Coverage   68.50%   66.50%   -2.01%     
==========================================
  Files         134      195      +61     
  Lines        3553     7538    +3985     
  Branches      867      867              
==========================================
+ Hits         2434     5013    +2579     
- Misses       1119     2232    +1113     
- Partials        0      293     +293     

Flag	Coverage Δ
backend-e2e-test-ginkgo	65.01% <30.00%> (?)
backend-unit-test	50.64% <70.00%> (?)
frontend-e2e-test	60.20% <ø> (-8.31%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
api/internal/utils/utils.go	57.39% <57.14%> (ø)
api/internal/conf/conf.go	65.30% <100.00%> (ø)
web/src/components/LabelsfDrawer/LabelsDrawer.tsx	3.92% <0.00%> (-80.40%)	:arrow_down:
...b/src/pages/Route/components/DataLoader/Import.tsx	4.91% <0.00%> (-78.69%)	:arrow_down:
...es/Route/components/DataLoader/loader/OpenAPI3.tsx	33.33% <0.00%> (-66.67%)	:arrow_down:
...pages/Route/components/Step1/MatchingRulesView.tsx	25.00% <0.00%> (-54.17%)	:arrow_down:
web/src/pages/Route/transform.ts	52.91% <0.00%> (-27.52%)	:arrow_down:
.../src/pages/Route/components/Step1/ProxyRewrite.tsx	70.68% <0.00%> (-24.14%)	:arrow_down:
...b/src/components/Plugin/UI/referer-restriction.tsx	69.69% <0.00%> (-21.22%)	:arrow_down:
web/src/pages/Route/List.tsx	57.50% <0.00%> (-18.13%)	:arrow_down:
... and 73 more

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