feat: Enable keepalive-time for healthchecks in VS and VSR

[ciarams87]

Proposed changes

Keepalive connections for health checks – The new keepalive_time parameter to the HTTP health_check directive enables keepalive connections for health checks and sets their lifetime. Establishing new connections is computationally expensive, so reusing connections can reduce CPU usage considerably when there are large numbers of upstream servers.

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 main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[jasonwilliams14]

I think it looks good. We can set a default between 30-60 seconds.
we can also give customers to disable entirely if they choose to.
it is a great new feature that will improve performance and HC effectiveness.

[codecov-commenter]

Codecov Report

Merging #3451 (718fa42) into main (f135169) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3451      +/-   ##
==========================================
+ Coverage   51.95%   51.99%   +0.03%     
==========================================
  Files          60       60              
  Lines       16806    16811       +5     
==========================================
+ Hits         8732     8741       +9     
+ Misses       7777     7775       -2     
+ Partials      297      295       -2     

Impacted Files	Coverage Δ
internal/configs/version2/http.go	0.00% <ø> (ø)
internal/configs/virtualserver.go	95.07% <100.00%> (+0.01%)	⬆️
pkg/apis/configuration/validation/virtualserver.go	94.39% <100.00%> (+<0.01%)	⬆️
internal/k8s/configuration.go	95.79% <0.00%> (+0.36%)	⬆️

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