feat: allow to use env variables in config

[ikatlinsky]

Why submit this pull request?

• [ ] Bugfix
• [x] New feature provided
• [ ] Improve performance
• [ ] Backport patches

What changes will this PR take into?

This ticket adds an ability to use environment variables in the dashboard configuration using viper decode hooks. Such change is required to simplify dashboard configuration based on the environment type. For example can be useful to provide dashboard users and their passwords.

Related issues

https://github.com/apache/apisix-dashboard/issues/2818

Checklist:

• [x] Did you explain what problem does this PR solve? Or what new features have been added?
• [x] Have you added corresponding test cases?
• [ ] Have you modified the corresponding document?
• [x] Is this PR backward compatible?

[codecov-commenter]

Codecov Report

Merging #2819 (14c292b) into master (e0290c8) will decrease coverage by 2.84%. The diff coverage is 87.50%.

:exclamation: Current head 14c292b differs from pull request most recent head 06577e2. Consider uploading reports for the commit 06577e2 to get more accurate results

@@            Coverage Diff             @@
##           master    #2819      +/-   ##
==========================================
- Coverage   77.49%   74.65%   -2.84%     
==========================================
  Files         139      201      +62     
  Lines        3746     7860    +4114     
  Branches      881      881              
==========================================
+ Hits         2903     5868    +2965     
- Misses        843     1681     +838     
- Partials        0      311     +311     

Flag	Coverage Δ
backend-e2e-test-ginkgo	65.16% <87.50%> (?)
backend-unit-test	50.10% <87.50%> (?)
frontend-e2e-test	77.44% <ø> (-0.06%)	:arrow_down:

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

Impacted Files	Coverage Δ
api/internal/conf/conf.go	67.87% <87.50%> (ø)

... and 62 files with indirect coverage changes

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[ikatlinsky]

@Gallardot fixed the failing unit test, please approve workflow execution

[ikatlinsky]

@juzhiyuan @bzp2010 @guoqqqi Sorry guys, can we proceed with this feature? Your reviews are very desired, thanks.

[ikatlinsky]

@juzhiyuan @bzp2010 @guoqqqi up for review, thanks

[juzhiyuan]

Hello, I personally not available those months.

[ikatlinsky]

@bzp2010 @Baoyuantop hi, hope you will be able to check proposed changes, thanks

[juzhiyuan]

I have to close this PR. Refer to https://github.com/apache/apisix-dashboard/issues/2981