feat: Enable setting mandatory and persistent in upstream healthchecks in VS and VSR

[ciarams87]

Proposed changes

See mandatory-health-checks

The mandatory parameter requires every newly added server to pass all configured health checks before NGINX Plus sends traffic to it. When combined with slow start, it gives a new server more time to connect to databases and “warm up” before being asked to handle their full share of traffic. Mandatory health checks can be marked as persistent, so that the previous state is remembered when reloading configuration.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork

[codecov-commenter]

Codecov Report

Merging #2251 (0655f94) into master (98239fc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 0655f94 differs from pull request most recent head 9b6e8c3. Consider uploading reports for the commit 9b6e8c3 to get more accurate results

@@           Coverage Diff           @@
##           master    #2251   +/-   ##
=======================================
  Coverage   53.41%   53.42%           
=======================================
  Files          43       43           
  Lines       13412    13419    +7     
=======================================
+ Hits         7164     7169    +5     
  Misses       6020     6020           
- Partials      228      230    +2     

Impacted Files	Coverage Δ
internal/configs/version2/http.go	0.00% <ø> (ø)
internal/configs/virtualserver.go	95.22% <100.00%> (+0.01%)	⬆️
pkg/apis/configuration/validation/virtualserver.go	96.29% <100.00%> (+0.01%)	⬆️
internal/k8s/configuration.go	95.65% <0.00%> (-0.39%)	⬇️
...ternal/k8s/appprotect/app_protect_configuration.go	86.74% <0.00%> (+0.57%)	⬆️

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[pleshakov]

🚀 (with some doc related suggestions)

[soneillf5]

@ciarams87 sorry for the late review