support configuring multi_accept directive via configmap

[tomxor]

What this PR does / why we need it:
Adds the enable-multi-accept ConfigMap setting which controls the multi_accept directive in nginx.conf file, instead of the current hardcoded value. Default is set to true.

Which issue this PR fixes:
fixes #2684

[ElvinEfendi]

/lgtm
/approve

[tomxor]

I'll squash once the build is green

[codecov-io]

Codecov Report

Merging #2896 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2896      +/-   ##
==========================================
- Coverage   47.56%   47.55%   -0.01%     
==========================================
  Files          76       76              
  Lines        5483     5484       +1     
==========================================
  Hits         2608     2608              
- Misses       2540     2542       +2     
+ Partials      335      334       -1

Impacted Files	Coverage Δ
internal/ingress/controller/config/config.go	98.27% <100%> (+0.01%)	⬆️
internal/watch/file_watcher.go	80.76% <0%> (-3.85%)	⬇️

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[tomxor]

pushed, thanks @ElvinEfendi !

[ElvinEfendi]

/lgtm
/approve

Thanks @tomxor !

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ElvinEfendi, tomxor

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