feat: support environment variable in config file

[nic-6443]

Sorry, the #713 closed because I wrongly rebase the upstream code, so now I have initiated a new PR. e2e has been added, can be review again

Please answer these questions before submitting a pull request

• Why submit this pull request?

• Bugfix

• New feature provided

• Improve performance

• Backport patches

• Related issues
  request help: Apisix ingress controller config.yaml couldn't read os environment variable #710

[nic-6443]

@tao12345666333

[tao12345666333]

Please resolve the conflicts, thanks!

[codecov-commenter]

Codecov Report

Merging #745 (2daf86c) into master (4a862e2) will increase coverage by 0.09%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #745      +/-   ##
==========================================
+ Coverage   31.70%   31.79%   +0.09%     
==========================================
  Files          66       66              
  Lines        6640     6652      +12     
==========================================
+ Hits         2105     2115      +10     
- Misses       4280     4281       +1     
- Partials      255      256       +1     

Impacted Files	Coverage Δ
test/e2e/e2e.go	100.00% <ø> (ø)
pkg/config/config.go	65.00% <85.71%> (+2.50%)	⬆️

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[tao12345666333]

The code LGTM. Thanks!

But I personally prefer to directly support configuration through environment variables or through configuration files
Instead of supporting environment variables in the configuration file.

WDYT? @gxthrj @tokers @lilien1010

[nic-6443]

The code LGTM. Thanks!

But I personally prefer to directly support configuration through environment variables or through configuration files Instead of supporting environment variables in the configuration file.

WDYT? @gxthrj @tokers @lilien1010

@tao12345666333 In the case that the user chooses to use file configuration instead of commands of container, the use of environment variables is to write sensitive information through downward api and secret. So if the program directly uses env, we need to consider the problem of file configuration and env configuration merging, doesn't make this problem easy.

[tokers]

The code LGTM. Thanks!

But I personally prefer to directly support configuration through environment variables or through configuration files Instead of supporting environment variables in the configuration file.

WDYT? @gxthrj @tokers @lilien1010

Softwares like ETCD and Istio support using environment variables directly, and the precedence is highest, so it's my understanding that we do should support environment variables, configuration file and command line options.

[nic-6443]

@tao12345666333 @tokers
Should we merge the current feature first, and add more options later?

[tokers]

@tao12345666333 @tokers Should we merge the current feature first, and add more options later?

Agree, or this PR might be too large to review.

[tao12345666333]

Let's move forward, this feature also has some usage scenarios.

[tao12345666333]

ping @gxthrj @tokers for review & merge.

[lilien1010]

🎉