feat: support environment variable in config file

[nic-6443]

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

[codecov-commenter]

Codecov Report

Merging #713 (ba75e7b) into master (a01888b) will increase coverage by 0.08%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #713      +/-   ##
==========================================
+ Coverage   32.36%   32.45%   +0.08%     
==========================================
  Files          65       64       -1     
  Lines        6488     6499      +11     
==========================================
+ Hits         2100     2109       +9     
- Misses       4137     4138       +1     
- Partials      251      252       +1     

Impacted Files	Coverage Δ
pkg/config/config.go	85.91% <85.71%> (-0.53%)	⬇️
test/e2e/e2e.go

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[tao12345666333]

can you add an e2e test case to cover this?

[nic-6443]

can you add an e2e test case to cover this?

OK