feat(webhook): make app refresh operation concurrent (#14269)

[phanama]

The current webhook handler processes all applications sequentially, with each iteration potentially calling the network-bound argo.RefreshApp() which calls k8s API to patch a refresh annotation. This makes webhook slow in scenarios involving hundreds of apps.

This PR introduces concurrent application processing to webhook HandleEvent(). We also expose a new configurable setting in ArgoCD CMwebhook.maxConcurrentAppRefresh to be able to control the degree of concurrency.

Fixes #14269

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
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• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
  * [ ] I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
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[codecov]

Codecov Report

Attention: 23 lines in your changes are missing coverage. Please review.

Comparison is base (99a4bf0) 49.67% compared to head (a753b47) 49.76%.
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #15326      +/-   ##
==========================================
+ Coverage   49.67%   49.76%   +0.09%     
==========================================
  Files         267      267              
  Lines       46383    46442      +59     
==========================================
+ Hits        23039    23112      +73     
+ Misses      21084    21068      -16     
- Partials     2260     2262       +2     

Files	Coverage Δ
util/helm/cmd.go	35.21% <100.00%> (+1.14%)	⬆️
...cationset/controllers/applicationset_controller.go	60.39% <0.00%> (-0.11%)	⬇️
util/settings/settings.go	51.90% <76.92%> (+1.79%)	⬆️
cmd/argocd/commands/project_role.go	0.00% <0.00%> (ø)
pkg/apiclient/grpcproxy.go	0.00% <0.00%> (ø)
util/webhook/webhook.go	74.01% <87.71%> (+5.35%)	⬆️

... and 1 file with indirect coverage changes

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[phanama]

added the comment based on ref:
https://github.com/argoproj/argo-cd/pull/4699/files#r514417202