Add helm admission hook

[burmanm]

What this PR does:
Adds missing AdmissionWebhookConfiguration for the mutating webhook

Which issue(s) this PR fixes:
Fixes #1145

Checklist

• Changes manually tested
• Automated Tests added/updated
• Documentation added/updated
• CHANGELOG.md updated (not required for documentation PRs)
• CLA Signed: DataStax CLA

[sonarqubecloud]

Quality Gate failed

Failed conditions

5.8% Duplication on New Code (required ≤ 3%)

See analysis details on SonarCloud

[codecov]

Codecov Report

Merging #1146 (aefb91c) into main (02d4415) will increase coverage by 0.07%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1146      +/-   ##
==========================================
+ Coverage   56.87%   56.95%   +0.07%     
==========================================
  Files         100      100              
  Lines       10300    10309       +9     
==========================================
+ Hits         5858     5871      +13     
+ Misses       3931     3928       -3     
+ Partials      511      510       -1     

Files	Coverage Δ
apis/stargate/v1alpha1/stargate_types.go	93.54% <ø> (ø)
pkg/stargate/deployments.go	92.96% <100.00%> (+0.16%)	⬆️

... and 5 files with indirect coverage changes

[olim7t]

LGTM

• installed two clusters with Kustomize and Helm side-by-side and checked that the configuration is the same
• created a K8ssandraCluster in the Helm cluster and checked that it started